I have Toontrak's EZ Drummer and Superior Drummer, in addition to Sonar's Session Drummer, but I probably use EZ and Session Drummer the most when developing ideas.
Platinum Samples has come out with several excellent expansion kits for Session Drummer, so I do gravitate towards it sometimes for those sounds. Also, EZ won't allow you to raise or lower the pitch of each individual drum, but Session Drummer will.
I use EZ or Session Drummer to quickly lay out new ideas, and sometimes I am happy enough that I stick with those for the final product. Often, though, I'll switch to Superior Drummer for a more polished final product.
Superior Drummer is a much more involved program, and it lets you mix-and-match kit pieces from any EZ Drummer & Superior Drummer library. Superior also allows you to alter the pitch of each kit piece and mess with the envelope, if you like.
Since you already own EZ Drummer, I'd suggest you play with both EZ and Session Drummer for awhile and decide which fits your workflow best. You can use MIDI drum patterns from EZ for Session Drummer and vice-versa.
Of the two, EZ Drummer probably has the edge, but Session Drummer gives tuning flexibility if you want it.