I use a MK16/S550 and love it! The MK16 is a diaphram first stage and the MK20 is a piston. They are both balanced. Supposedly, the balanced piston performs better on computer testing, but the difference is minimal on computer testing, and undectectable in recreational diving. The main difference I find between the two is I can be lazy about rinsing a diaphram reg. Water actually flows into a piston reg, while on a diaphram, the reg is sealed (ambiant pressure is on the diaphram). So...I can wait a few dives before soaking my gear without too much worry.
One thing about the R380. I'm sure your dive shop will tell you, but the R380 second stage is set in the "storage mode". There is a little nob on the front of the face plate that has to be switched to active mode..or else the reg will free flow as soon as you get in the water. Reason for this: the storage mode takes the pressure off some little bit (don't you love the techincal terms!) and keeps the other little bit from wearing out. Once you switch it though, it work just fine.
Hope this helps!