Regarding the 2 drums, I've never played the T4, but the two kits I mentioned that were demoed sounded extremely warm and realistic compared to my S910's drums. I haven't given a good listen to other kits. I love the drums in all the T4 demos, but I don't know what kits they're using.
As Ian said, you can substitute a custom drum kit as well on the T4, so ...
Anyway, I look forward to getting my S950. If it's more than a few steps above the S910, I'll be pretty happy.