Actually, its an R190, not a R180.
I have the R380 and R190. They are identical on the inside and take the same annual service kit. They seem to breath about the same to me. I like the R380 because it's nice and small but, the R190 is easier to take apart underwater. Either one would be fine though.
As for the S600, that's what I use as my primary. No complaints at all. I can't imagine anything breathing better. Very reliable and easy to service. Great reg, can't recommend it enough.
The X650 has not come out yet. ScubaPro is behind schedule on that one. A buddy of mine has one on order but, I doubt that he will see it anytime soon. The X650 is just the updated D400, a reg that a lot of people really like.
Honestly, I don't really think that you would be able to tell the difference between a S600 and a X650. The X650 can't be taken apart underwater though. Either one would breath great though.