You may be better off running high octane simply because it gives you a hedge against detonation. If you engine never detonates under any circumstance, then it just won't mattter. It won't hurt, but it won't help either. If your engine may detonate sometimes, then it can help.
It's important to note that higher octane fuel will never, by itself, make more power. High octane fuels burn slower, and delay the onset of detonation as noted. This may allow you to bump up the compression ratio, or turbo boost, nitrous etc. (not likely in a chainsaw;^) ) which will make more power. The HO fuel is simply allowing you to do these other things without destroying the engine.
Ethanol is another story. I am not a big fan of the blended fuels, especially for small engines which cannot automatically compensate for them the way a modern EFI auto engine can. Alcohols are a bit corrosive, especially methanol which is not used in blended fuels. Again, small engines have a lot of parts in the fuel system which can corrode in contact with alcohols. EFI autos are pretty much all stainless steel wetted parts so this does not matter much for them.
I am a big fan of the product Sta-bil. It defintely extends the life of fuel to at least a year. I recently dusted off my generator after 5 years in my garage without starting. I drained the Sta-bil treated fuel from the tank and bowl (no varnish at all, just old fuel) added new gas and it started in one pull!
Jimbo
