First rule of music software - no one has a monopoly - all software has one good sound. More is therefore better, but what if you have to choose?
I have used Groove agent for 10 years. It does everything I want but I was getting in a rut with it so I just bought EZ Drummer 3.
Both software I paid for, so I am impartial
Which should you buy?
You rarely write a song from drums first so finding a groove to match your song is important. With GA it's trial and error, previewing one at a time. EZ has BandMate/Tap2Find that makes your workflow so much faster. Just drop some MIDI and it will analyse and suggest matching groove. This saves a fuck load of time - if you have some keys or a bassline in MIDI which I often do, usually starting with keys or a rhythm guitar track then adding keys next.
Why obsess about drum tracks?
OK, the default drum pattern is Kick on 1 and 3 and snare on 2 and 4. That'll get your track started but, if you want to breathe life into a track you need more options.
Steely Dan, those audiophile magicians, knew how important a drum groove was, they hired the best session drummers of the day (Gadd, Purdie etc.). These guys turned up to a session in an artic truck with roadies to set up. A couple of in demand guys even had two trucks and kits so they could do multiple sessions in a day!
My only caveat, the drummers they use on both GA and EZ were not Vinnie Coilutas, decent, just not premier league.
Comparisons
Both have search (genre, time signature etc.)
BPM only matters insofar as some grooves only work close to their intended bpm so more patterns at all bpm's is an advantage. If you want patterns EZ has more and better
Editing EZ has a built in MIDI editor though you can easily use the Cubase midi so perhaps redundant.
Kits - Groove Agent has more kits, though you can customise the three kits in EZ with different drums and cymbals making it effectively limitless..
Conclusion
Which should you buy?
For Interface and groove quantity EZ wins but Groove Agent is free (with Cubase). Both can do the job. If you can, buy both!