What are the differences between these two racquets? 1. In terms of fragility? The feedback in tw say they break easily. 2. In terms of feel? The Yonex RDX500 is perfect for me, except the whole racquet face is the sweetspot and I don't feel the difference when I'm off centre until I frame one. I wanted my next racquet to be just as arm friendly as the RDX500 but with a little more crisp feedback. 3. In terms of comfort? (my problem is in the elbow) Any other differences? Is the extra money for the new Ki 5 worth it? Thanks for all the help, I cannot demo due to location.