All of these are really great questions to ask and I’ll leave it to the community to offer their impressions.
The first question I would ask yourself is how important is spatialization to your game experience? What are your goals and how will spatialization feature in your project?
Steam Audio uses game geometry to build an acoustic impulse (using physical fluid simulation) which means that its goals lean toward creating an acoustic simulation.
Google Resonance’s goals are to bring binaural audio experiences to mobile devices, so their aims try to achieve fixed costs on the processes that normally explode CPU budgets per source.
Oculus straddles the middle-ground. Their goals are more focused on simulating spatialization and not as heavy in creating physically simulated acoustic environments but rather algorithmic approximations of spaces.
Because of these differing goals all of their spatialization algorithms will sound different and on some, certain features might not be available.
Finally, not all plugins will return the audio back to Unreal, so in some cases, it will be difficult to mix and match certain features; in other cases, some features depend on other features to be activated down the signal stream; in yet other cases, mixing and matching totally is fine.
Platform is another consideration. Not all plugins support all of the other platforms, some platforms don’t currently have anything, and some platforms have exclusive systems.
So my recommendation is to look at the project itself, what are you trying to achieve in terms of spatialization, occlusion, reverb, etc.
The built-in panning and occlusion may be simple, but are low-cost and effective in most use-cases. The built-in reverb sounds really great.
As far as demo maps, I’m working on it. 