Comps are double butted (2.0/1.8), Super Comps are Triple (2.0/1.7/1.8). Revolutions are usually 2.0/1.5 although there are 1.8/1.5s out there.
CX-Rays (and Aerolites) have a higher fatigue life than round spokes, and a slight aero benefit. Nowt else. Not worth it on the MTB unless you break a lot of spokes. If used in conjunction with alu nips the nipple breaks instead of the spoke, which is just as much faff. I broke a number of nipples while using CX-Rays/Aerolites (I’ve had 2 sets of MTB wheels with the former, one with the latter).
Alu nips save weight, but will corrode and seize, you can oil them etc to put it off, but it will happen eventually. I’d not bother unless it’s a super light wheelset (I still use them on my race wheels FWIW).
Buy DT spokes from Action Sports, far far cheaper than the UK shops, they sent me 10 spare Revolutions when I bought from them too.
Super Comps are a safe bet if you want light, but don’t want to go as fragile as Revs. Saying that a decent build on Revs would be tougher than a shonky build on Super Comps! Personally I don’t really see the point. If you want light go for Revolutions, if you want cheap go for Competitions, not that sure about the middle ground! YMMV.
There’s a replacement for the Arch coming next year too apparently, if you’re in no rush I’d probably wait for that.