The 35 will run cooler.
A smart-assed engineer could build a 28 that would refute that claim, but it would have to be longer, and probably heavier, than the 35 motor it competes with. I doubt there are any such critters on the market.
Randy, it's not a question of "does it have the juice?" or "can it make the power?" -- the answer to both is "yes". The question is "how hot will it get while doing so?", and the answer is "more than a bigger, heavier motor."
I really wish that we had a herd of science-minded teenagers on tap here, because I think that another set of questions to ask (and get the answer to) is "how much advantage is there to more aggressive cooling measures?" and "what's the most efficient way to cool an electric motor in a model going 55 miles per hour?". But -- I don't have time. It'd be a good science fair entry, or extra-credit project for a high-school physics class, or possibly even a good chunk of a senior-year project if you happen to be doing aeronautics or mechanical engineering.