Correct me if I'm wrong, but wouldn't two-cycle be superior for running an engine on many steep slopes? And is it still a fact there aren't any?
As for what mower to choose, ask yourself what longevity you want out of it. I'm in the market too and would really like a Honda HRX217HYA. This is the Cadillac of self-propelled walk behind homeowner mowers.
But, the thing costs $700 bucks. It might last forever, but I don't want to mow forever!
Read the posts here. There are a lot of good designs out there, a lot of crappy ones too. Determine your budget, then research at that price point. And look for money back satisfaction guaranteed. And good luck!