My LDS suggested the Mk25, because it can be easily dialed in right there on the spot.
I have no idea what they're talking about, except for the user adjustable inhalation resistance on the S600. I believe, though, that the SP spec is to adjust the reg breathing resistance with the adjuster all the way out, which means (theoretically anyhow) that you can only make it harder to breathe, not easier. Certainly the cracking effort on the S600 could be set a little lighter than on the R190, and since the spring is lighter, it does theoretically take less effort to keep the reg flowing.
One real difference between the two is that as the tank empties, the IP on the MK2 will drop, eventually to the point where you will feel a difference in breathing resistance. I have found from experience that I can tell my MK2 is breathing harder at about 300-400PSI, and the deepest I've ever been at that point was 25 ft sitting on a platform breathing the reg down to see what happened to resistance. My feeling is, if you're at any real depth with 300PSI, chances are you are unaware of your dilemma and the slight increase in breathing resistance could be a welcome reminder that you're soon to be in a pickle.
I'd still be diving my MK2 all the time except I got interested in older SP piston regs, just out of curiosity. One thing I've found is that when in good shape, they all breathe great. One of my favorites is the old metal case 2nd stage, the 156. If you like simplicity and want a higher performing reg than the MK2/R190, you could buy a used MK5/156 (or 109) and get someone to rebuild it. They're not quite the paragon of simplicity that the MK2 is, but they're pretty close. Bet your LDS thinks that's a bad idea!
