SD is a different engine than EZ. You can run EZ samples on SD but not the other way around.
The sample packs are huge if you install the room mics and Decca tree samples and all the other bells and whistles. Like 30 gigs plus.
The mixer section is way more expanded compared to EZ, and has FX inserts and included FX, and has bleed control too on every instrument.
There is also a "construct" tab and under that setting you can modify the drum sounds with an envelope, and choose how each drum is struck. Looking at the "articulation" for high hats in Roots Sticks there are 25 options for how the hats are struck. Fewer options for other stuff like snares and toms, but there are a crazy amount of sounds available in the SD packs.
Under that same tab there is a MIDI learn option if you are using pads or a keyboard. All you do is select the drum and hit the pad or key and it will be assigned to that controller.
I have a bunch of the EZ packs and the NY Avatar and Roots Sticks SD packs. The Roots pack has been my default "kit" since I got it.
TT has another bit of software called EZ Player that is a MIDI editor. I'm not sure if it is included with SD2 or not. I picked it up separately when they were selling it for $49. It is way easier to edit MIDI loops than in a piano roll view IMHO. Drag and drop the loops into the player and you can shorten them, edit them to combine loops, save the new loops, all that jazz. It will map wrap loops from other programs too like Slate, Roland, Yamaha, etc.
The one thing I don't like is the small imaging of these apps. You can't push the image to full-screen size so in EZ Player only 10 bars are visible at a time.
I did have some problems installing Roots. One of the DVD's was corrupted and it would hang the install. I called Sweetwater and they sent me a box set with the entire SDX/EZX catalog. Just need to buy a license for any given set and it's good to go.
If you can get SD2 on a blowout sale and a couple of the sample packs on sale it's a pretty good deal and the samples are well done. When I bought SD2 it was cheaper to buy it outright than to buy the upgrade from EZ. The good thing is they put the engine and the SDX's on sale a few times throughout the year.