My.02 on the difference. I really should have ask how your going to fly it. If your going to do pattern/sport style flying. Then either engine would be a good choice. However for instant acceleration and torque the Saito 1.00 is the way to go. Swinging a 15x4W APC prop on 15% fuel that thing would be awsum. The .91 will swing a 14x4W or a 14x6MA but the difference in the way the it pulls the plane will be noticable. It's just hard to beat the instant power and ease of tuning and starting of a modern four stroke. As a bonus the stock Saito muffler can be turned to be barley noticeable. That will save you $50. for a pitts to fit the .91.
Neither of these engines will give it unlimited verticle, but for my style of flying the 1.00 would suit me best.

David