Robus pretty much hit the nail on the head about Jamstix. It has the possibility to be a great program, but it's complicated, & has a few quirks I personally don't like (but I have found a couple/few work-arounds for)...
I've never used EZD2, but I have EZD1 & Superior, & there's no comparison between those two IMHO. Superior's sounds are much better (to me anyway), & you have much more control over 'em, but the thing about drum vsti's that actually make them sound real is the performance/midi, which brings me back to Jamstix...
One of the quirks I'm talking about is I could never get Jamstix to map a custom kit for Superior. So, I re-mapped Superior to fit the midi Jamstix spits out. I shouldn't have to do this, but, I do. Some folks are able to get it to do this, but I'm not one of 'em. But the work-around I made for myself does a pretty fair job, IMO...
Another thing is the fills (probably the biggie to me), sometimes it nails it, other times, I have to keep going over & over it. But, the good thing about this is you can save those fills, then re-import them & edit to your heart's content...
I never use the Jamstix internal sounds, I just could never bond with them. They're not terrible, but the sounds in my other drum vsti's are much better IMO, YMMV...
Jamstix 4 is supposed to be coming out soon, but I really don't know what's supposed to be upgraded. If they fix some of the issues I have with it, I might consider the upgrade, if not, then I won't...
I've been using Jamstix for about 2 years now too, & have barely scratched the surface of what it's truly capable of, so these quirks I mention could very well be user-error on my part, I dunno. I will say it's a deep, complicated program, but it does a better job at behaving like a drummer than any midi loop you'll find, just getting it to do what you want it to takes time to learn...
Not trying to sway you either way, but I'd wait for Jamstix 4 & see what the changes are myself. I've also heard that Superior 3 is coming this year, dunno how true that is, but that may be something to look into also, if it happens...
I'm sure EZD2 & the current version of Jamstix 3 work together fine, but I don't know that from experience...