Mike,

The ST91 will be just fine for your needs with some throttle management which means if the nose is pointing down even slightly, pull back on the throttle as it will be quite a bit faster than the 4 strokes. 4 strokes turn bigger props but at lower RPM's so they do not fly a plane very fast, just make it pull vertically like gravity was a passing fancy.

A two stroke is built to run high rpms so they need a smaller prop which than a 4 stroke will increase overall flying speed.

You will be overpowered but fuel consumption will not be bad as you'll be flying around at half throttle pretty much most of the time.

The ST75 will be somewhat limiting but will do just fine. I'd go with the 91 just for future use if not anything else

I was only suggesting the four stroke should you ever get to thinking about it and did not mean to confuse the subject, sorry