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*