I missed this discussion (red it only now).
It is interesting. I think the key will be the price. There are now, in the market, two prices for engines:
1) "low cost" (even if engine is good). 100->250 euro. Good engines, but missing (or primitive) of some key tools (material editor, world editor, etc...). Or not cross platform.
2) Unity engine, in my opinion (I used Unity for some time, and even completed a game: http://www.massivecreek.com/home) , but the engine self is missing of many features (no LOD, real-time shadows are not so... good, missing HUGE terrain support, etc...) But Unity has an un beatable IDE: flexible, works well, customizable, etc... (Win & Mac).