I don't use sampling that much, but with sampling like the SPD-SX has, you can connect a sound source directly to the unit and record those sources, from the 'audio in' connectors (LOL at HUGE stock pic):
With sample playback, you can load samples like from a thumb drive, but the unit itself doesn't actually sample. The Yamaha Multi-12 can do that.
Pretty sure with phrases, you hit record, then play the unit using the internal samples, and that recording can be saved and played back by hitting a pad.
I've got an SPD-30, which can do single or layered phrases, but not sampling or sample playback.
My Yamaha DTX-Multi can do sample loading and playback, and record (single, not layered) phrases, but not actual sampling.
If you want actual sampling, you'd have to get the SPD-SX.
About the pads, the Roland spd-30 can take 4 pads, but each can be dual trigger, so it can have 8 externally triggered sounds.
You could probably use your old Simmons pads, and they'd most likely work with at least one zone.
AFAIK - all pads on all units can be programmed to use any available sounds, and all can use clicks. With kick pad and hi hat triggers they all can be used as small kits.
I have my spd-30 and multi-12 set up as small kits.
You can't load keys or bass sounds into the spd-30, but you can use whatever is available from the stock sounds.
The Yamaha really excels in this area. You can load up to 4 layered sounds per pad and velocity switch between them, as well as a lot of other programming tricks.
As to feel, the spd-30 pads are firmer than the multi-12's pads, but I use the multi all the time with no problems. Can't speak for the spd-sx because I haven't used it.
I think that about covers the questions in your post.
But I strongly
suggest that you download all the manuals to see which one best meets what you want. And if possibe, try them all out first before deciding.
Also a little general reading about sampling and sample playback/triggering would probably also be helpful.