I own both racquets and like both for different reasons. They are both very solid racquets. I feel the K 90 is a little better on grounstrokes, possibly because of the added weight. It feels a little more solid. The advantage the K Blade has when hitting groundstrokes, is if you are playing someone that can keep pressure on you with depth and/or pace, you need to have quite a bit of strength with the K 90 to get out of jail. When you are on the run and reaching for a return from around the baseline, it is also difficult to flick your wrist with the K 90 and get descent pace on the ball. As long as you have time to set up, both racquets are very good. Also, when hitting into a descent head wind, it is a little more difficult to get depth on your shots with the K 90. Lastly in regards to groundstrokes, I can generate a little more spin with the K Blade when needed.
On volleys, I slightly prefer the K Blade. Both racquets are very solid with lots of feel and touch, but the K Blade is more manueverable and the additional head size helps.
On serves and overheads, I like the K Blade. I feel I can generate more power with less effort. It just seems to swing easier than the K 90. I'm sure because of the lightness. Both racquets are among the most accurate servers I have ever used. Obviously you will still miss your spot more than you want, but everything will be closer to your spot than with most other racquets. Also, while serving as with other shots, I can impart spin easier with the K Blade.
Probably the determining factor in favor of the K Blade (IMO) is the forgiveness. The K Blade gives you much more forgiveness than the additional 3 sq. inches would indicate. This factor alone is the reason I primarilly use the K Blade. While playing heavy hitters, the K 90 is demanding when you don't have your best game.
As with all these opinions, take them with a grain of salt. Everyone feels somethng different when playing the same racquet. Also, I am still fairly strong physically, but I am 48 and not as strong as I once was. If you can manage the weight and smaller sweet spot, the K 90 may be a better choice for you. Either way, I feel both racquets are among the very best players racquets currently on the market and the differences I noted are very marginal. Good luck.
