I built mine stock and at the time I was having the same dilemma. Saito 100 or 91?
Keep in mind that GP calls for an OS70 at the most for a 4stroke. I was also trying to keep the weight down and although the 100 is more powerful than the 91 and about the same weight (but heavier) the 91 is a lot more powerfull that the OS70 and about the same weight or lighter to boot. Now Factor in the considerable more expense for the 100 versus the 91 and I think the choice is clear ( at least it was for me at the time....I got a 91 for $200) I started flying with a 14X6 and switched to a 15X4W for 3D. Great combo.....it floats in as slow as you can dare. Watch out for the elevator though. Given the right combination of throw and airspeed and it will tip stall on you without too much warning.
I had to butcher up the cowl pretty good to fit the engine in and I had some overheating problems. I think my fuel line is too long and makes for a lean mixture but not absolutely sure.
Bottom line the 91 suits this plane perfect IMHO. Enjoy!