+1 to keep it, that would allow people to pick one explicitly rather than having to necessarily rely on some logic.

But will you be able to automatically detect which one to use by default? Like it used to automatically pick one for JBossStandAloneJtaPlatform - I'm not sure how that works but it's a nice touch.

Regarding extending an existing type: I guess it doesn't matter much. Feel free to extend it if that makes the code more maintainable.