I tried both before settling on the SW104. In stock form, they feel very dissimilar to each other. The blade 104 is significantly lighter and because of that, the sweetspot feels much smaller and off-center hits are very jarring. The SW104 also has countervail, which I do not believe the regular blade 104 has. I have not tried the same racquet with and without countervail so I'm not exactly sure how it changes the feel, but the SW104 definitely has a more solid and vibration free, almost wood-racquet-like feel when hitting the middle of the sweet spot. The blade 104 had less of a pocketing feel and was more springy, but that could have just been differences in the demo racquet strings (synthetic gut in both cases). The SW104 feels stiffer overall, which I prefer.
I didn't notice much difference in length between the two. The SW104 seemed to give about 5% more ball velocity, but it is hard to know how much of that 5% was due to swingweight and how much due to the strings. If I had to guess, I'd probably say the vast majority was due to the racquet.
I'm about a month into using the SW104. I have 5 grams of lead at 3 and 9 o'clock and about 25 grams added to the handle area. It feels like I want a bit more swingweight for groundstrokes but even as it is now, it's almost a bit too unwieldy for higher 4.5 level doubles. Overall, for singles where I play an aggressive, attacking game, I usually try to not have a difficult volley and if I hit a poor approach shot and flub the volley off a good groundstroke by my opponent, I'm okay with that.
I'm 57 years old, USTA rated 4.5, and still can hold my own at 18 and over 4.5 singles. The SW104 has brought some extra reach and ballspeed to my game that has helped quite a bit. Oh, one-handed backhand as well, and I can hit a ton of top from that side as long as I have some time to get my feet planted.