I'm not a developer on either of these platforms, though I do currently use a Pre, but will be moving to an Android phone (EVO) when it comes out.
Since I'm not a developer on these mobile platforms (I do embedded systems that aren't consumer oriented) I don't really have a good sense about what makes one better or worse than the other from a developer point of view and the technicalities of why, but I do know that at least the UI conventions used on WebOS are very nice and intuitive. The app-switching "card" metaphor is worth its weight in gold alone and is perfect for a tablet application with some tweaks (I think the iconography in the current WebOS is getting a bit stale and fisher-price looking, IMO)
That's sort of why I don't really see why WebOS couldn't just morph into another UI-skin for Android, particularly if the developer support is mainly going to split between Android and iPhone OS. It seems like a bit of a problem to have to try and be a third OS that needs to gain developer support to get an app-base that is attractive, particularly on a tablet. It seems the biggest problem with stock Android is its UI from what I read - something I have no experience with, so I'm obviously talking a bit out of school - and not its developer environment.