I have both Ez drummer 2 and Superior Drummer 3.
If you are just looking to mash out demos EZ drummer is more than good enough in my opinion.
That said, Superior really is superior in pretty much every way. There are libraries in ez drummer where the routing flexibility is limited quite a bit. Off my head the Rock Solid library has all the toms on one channel. This means leveling them in your daw isn't possible. In Superior not only is it possible to bus things however you want, you can take the bleed out in other drums, create midi from acoustic drums for use in superior, the plugins are great, and the routing is pretty much unmatched by any other drum software (I own a few).
Also: Unless it was changed in version 2 (I don't remember TBH) Superior is 24bit samples where as EZ drummer is 16bit. This pretty much doesn't matter at all but it's another thing to note.
TL/DR: EZ drummer is excellent, sounds good, and is easy to use. Superior Drummer 3 is superior in pretty much every way though and many of the features carry over. Also anything you bought for ez drummer can be opened in superior 3 so you don't lose those libraries.