...A Mk16/S550/R380 setup.
Firstly, it's important to choose the correct second stages... Your primary should be balanced, and your secondary unbalanced. This helps you to have one
really great performing regulator at depth, while the lower performance of an unbalanced reg helps to prevent freeflow.
Secondly, I liked the idea of the simple design in all of the above regs. Don't let all the knobs fool you... The missing adjustment on the S55 and R380 is a knob which "detunes" your regulator. Why would you ever want to detune your reg? A vane works better to help prevent freeflow anyway...
Thirdly, all of the above are diaphragm regulators, which tend to have several advantages... They are naturally sealed, which means that they're great for salt or muddy water (which I am in all the time), and they won't ice up like pistons will (a nonissue for me). With the Scubapro S600 "big brother," only silicone grease is what keeps things sealed up. I'd rather have the diaphragm.
Forthly, I can get a Scubapro reg worked on ANYWHERE. Their warranty is tops, and their reputation is the best there is.
Fifthly, not that I put all that much stock in it, but the recent "Mother of All Reg Reviews" from Scuba Diving magazine touted the S550/MK16 as both a "best buy" and a "tester's choice." It came recommended even over the S600.
Sixthly, no swivel turret on the MK16. Why would someone want that anyway? Doesn't it just make more sense to have a nice, solid setup back there? Can you imagine a "swivel valve" on your tank? Of course not... The idea is silly, and sounds dangerous. That's how I feel about swivel turrets. I wouldn't want anything to get turned around on me back there, anyway...
Lastly, did you see the price on this combo? I just bought an S550/MK16 combo from Leisurepro for $260, complete.
The only reason that I can see wanting the MK25 over the MK16 is the second high pressure port, in case you want to do the "transmitter with redundant SPG thing." But then you'd have to spend more money to get a 1st stage that has a swivel and is a piston design, which isn't sealed quite like a diaphragm.
Now, that said, I've heard of nobody having any problems with the MK25, even in harsh environments...
The Apeks? Nice regs... But the funny sized hoses are a little off... And comperable performance is twice the price in an Apeks...