ORIGINAL: Test005
From my (theoretical) point of view:
1. General sportflying, basic manouvers; .70 will be OK
2. For 3D flying, better/unlimited vertical performance; .91 is the choice
This question is as much "How do you fly?" than "What's the best engine for this plane?"
I'd classify my sessions as "General sportflying" and my needs, skills and lack of interest in aerobatics/3D doesn't call for excessive power, just enough to get off the ground and cruise around with the occasional loop, roll and fast-low-altitude fly-by.
For me buying a .91 and hardly use it over 60% would be a waste of $, while the .70 would be "boring" for the hardcore 3D pilot that need maximal *omphhh*
Well put Test005, that is exactly why I have a YS .91FZ on mine with a APC 15x6 prop. I flew a friend's 4*60 with a Saito 61 (I think that was it) and it was very very slugish, but I am use to the power of mine also, so I had to take that into account also. I think a .70 is the lowest I would go also.