Hey Vlad,
First of all, there is a topic about using the drum rack with Ableton. Some tips on how to set tings up and a preset made by another QFG student that allows for better routing in Ableton: Ableton Drum Effects Rack for finger drumming
So I think the above topic might answer question 2 (I’m not sure, I’m not an ableton user but I think this is what you’re looking for)
The difference between drum rack and AD2 or SD3 is the realism of the sounds. If I oversimplify things a bit, you could say that drum rack is a bunch of drum samples and AD2 and SD3 are drum simulators. So they have all kinds of stuff under the hood that make things sounds like a real drum kit when you’re playing. Things like multiple samples (if you hit a snare multiple times it’s never the same sample) and velocity layers (a loud hit is a sample of the drummer actually hitting hard instead of the volume just going up).
It makes a big difference, but whether it’s ‘better’ depends on what you like. It’s not like “samples” are bad. They just sound different than a fake ‘real’ drum kit. For me personally the creative juices start flowing when I feel like the kit responds to my playing in a more natural way and in those cases AD2 and SD3 (I prefer AD2) do a great job!