Swingweight is the primary determinant of both 1) inherent racquet power (also known as ACOR, Power Potential, or "bounce") and 2) how fast you can swing the racquet. Since the speed of a shot is a function of inherent power and swingspeed, swingweight is the most important racquet spec to understand.
Swingweight is roughly proportional to hittingweight, which is the proportion of the racquet's total weight that is 'represented' at a particular impact location. Hittingweight is almost directly proportional to Power Potential. TW University removes any uncertainty over this matter with their objective Power Potential testing, where they fire balls at a stationary racquet and measure the bounce in the real world. This is the last word on inherent racquet power. To see how well swingweight correlates to Power Potential (and how poorly static weight does) read this article:
http://twu.tennis-warehouse.com/learning_center/specsandspeed.php
Power Potential can also be estimated using a formula. This is not ideal, as the formula can't take into account such things are racquet flex, headsize, string pattern, etc. However, in the OP's example, since the two racquets he presents have identical flex, headsize, etc., the formula works perfectly to compare these racquets.
We can use TW University's customization worksheet to access the power potential formula, as it is built into this tool, here:
http://twu.tennis-warehouse.com/learning_center/customizationReverse.php
Plugging the numbers for the OP's Racquet A and B (OP doesn't specify how headlight, let's say 9 HL) we find:
Racquet A (10 ounces, even balance, 330 SW) Power Potential: 40.86%
Racquet B (12 ounces, nine points HL, 330 SW) Power Potential: 41.06%
You can see that the extra two ounces added to racquet B does increase the power potential moderately. It may even look like a lot, but let's add just 3 grams to Racquet A at 12 o'clock to see what happens:
Racqet C (10.11 ounces, 1.1 point HL, 339 SW) Power Potential: 41.83%
Adding just one-tenth of an ounce to the head of Racquet A (and thusly increasing swingweight by 9 units) increased power potential much more than adding two whole ounces in Racquet B.
This illustrates how much swingweight has to do with inherent racquet power and how little static weight does. This fact is the basis for the development of the "hammer" type racquet weighting, which is very popular on the WTA. The Williams sisters, for example, use light sticks with extremely high swingweights.
However, Racquet B, although only marginally more powerful than Racquet A, does have it's advantages. The extra weight in the handle increases recoil weight, which makes this frame much more stable on shots where you cannot generate high swingspeed, such as volleys and serves. Also, weight in the handle improves control when the racquet is swing very fast, as explained here:
http://www.racquetsportsindustry.com/articles/2006/04/racquet_handle_weighting_and_m.html
So now that we know that swingweight is much more important to
inherent power, we can move to the other parameter that determines actual shot speed - swingspeed.
Controlled scientific studies have shown that, again, swingweight is directly proportional to swingspeed. One study, using rods of different weights but the same swingweight, showed that static weight has no influence on swingspeed. So, a 12 ounce rod with swingweight of 330 could be swung as fast as a 10 ounce rod with swingweight of 330. This would suggest that Racquet A and Racquet B, since they have the same swingweight, could be swung just as fast as each other by the same player.
I think most of us have found that in the real world the 12 ounce racquet is going to swing slower than the 10 ounce one. There could be two scientific explanations for this:
1) In most of the studies, the swingweight used in the tests was the swingweight measured at the wrist axis, about 10 cm beyond the end of the racquet. Some studies even include the weight of the hand. When you measure swingweight at the wrist, there's a formula you use (the parallel axis theorem) that takes into account balance and static weight. So the 12 ounce racquet will have a higher swingweight
at the wrist than the 10 ounce racquet.
2) Swingweight, measured 4" from the bottom of the handle, or at the wrist, is a measure of how difficult it is to swing the racquet in an arc. But not all of the swingpath is circular. The initial part of the swing is often quite linear, and during this portion of the swing the heavier racquet would be more difficult to accelerate, even if it has the same swingweight. This is also well explained in the Racquet Sports Industry article above.
So, the short of it:
Swingweight is much more important to inherent racquet power than static weight.
Swingspeed is primarly dependent on swingweight, but if the swingweight of two racquets is the same (like with Racquet A and B) then the one with less static weight (Racquet A) will be easier to swing, and can thus be swung faster. But, it might be more difficult to control.
Finally, inherent racquet power is much more important on shots with a lower swingspeed. Beginners, who can't generate fast swings, can therefore use a very powerful racquet, as the racquet makes a bigger contribution to speed of shot when the swing is slow. This is also the case on volleys and service returns, where a short swing leads to slow swingspeeds.
On the other hand, at fast swing speeds, the speed of the racquet head is much more important to the speed of the shot than inherent racquet power. Inversely, this means that at fast swing speeds the inherent power of the racquet doesn't really matter. In this case, a racquet with better control is more useful than one with more inherent power. This also implies that if you want to hit the ball fast, you should chose a racquet that you can swing very fast, even if it isn't particularly powerful (in other words, a racquet with a low swingweight).
But then you get into the tradeoffs:
1) If you choose a low-swingweight racquet you will be able to swing it very fast, generating fast shots with lots of spin, BUT, on shots where you can't take a full swing (volleys and returns) the low-swingweight racquet will be unstable and not very powerful.
Racquets that provide this type of tradeoff are the lighter player's sticks, like the Volkl Power Bridge 9, Babolat Pure Storm Ltd and Dunlop 4D 100.
2) If you choose a racquet with high-swingweight, but low static weight (like Racquet A) you will have high inherent power and, because of the lower static weight, you will be able to swing it pretty fast. This will make for a powerful racquet from the baseline, reasonably stable and powerful on returns and volleys, BUT, because there is not very much weight in the handle of the racquet, control will suffer.
Racquets that provide this type of tradeoff are the baseline-oriented Pure Drives and hammer-type sticks. These racquets, as noted above, are vary popular on the WTA tour. Justine Henin uses this type of setup and it obviously works really well for her, even up at the net. Rafael Nadal swings this type of frame as well....
3) If you choose a racquet with high-swingweight and also high static weight (like Racquet B) it will also be quite headlight (because the extra weight has to go somewhere other than the head, which means the handle, which means the racquet will be headlight). The high-swingweight will confer good inherent power and stability at the baseline. The racquet will not swing quite as fast as Racquet A, but because there is lots of weight in the handle control will be very good. Stability on returns and volleys will be excellent because of the swingweight and high recoil weight. Maneuverability at net will be good because of the headlight balance. However, if the racquet is just a bit too heavy for you shots that require quick adjustments or wrist flicks will be tough to pull off. Also, if your strokes are wristy and explosive, the heavy weight might impede your technique and reduce the amount of spin you can generate. This type of stick is best for more classical technique and serve and volley play.
Racquets that provide this type of tradeoff are heavy player's sticks like Wilson Tour 90 and 95, Volkl PB10, Dunlop 4D 200 Tour.
Most of the men on tour use this type of stick, although they usually add more lead in the head, making the swingweight higher. This is an adaptation to today's power baseline style. Historically, Pete Sampras's stick was the perfect example of this at 13 ounces, 8HL and 365 swingweight.
