Did you listen to the demo's?

I haven't used either. This is what I gathered from their demos. I got a demo of GrooveAgent and I wasn't impressed. It may also depend on what you want to do. EZ drummer sounds more natural/realistic to me.

There is no right or wrong answer to this, it all comes down to personal preferences, try both of them out and see which fits you best. Groove Agent's strength is preset rhythms and variations, i haven't tried EZ Drummer to compare. ANother decent drum app for good value is Jamstix by Rayzoon.

I've used both.

They're excellent programs, however EZDrummer sounds that little bit better and I found it easier to use.