Thanks for the answers,
My first stage is MK11. To be honnest I dont much about it, I'm still at the bottom of the learning curve so can any of you explain in simple terms in which conditions and when the S600 would be beneficial.
Cheers
Scubajo007
The MK11 is (I think) the newer version of the MK16, which means it's a perfectly acceptable non-sealed diaphragm reg, very similar to the MK17 without the environmental seal, which by the way you would only need in extremely cold water. So, your first stage is fine at any recreational depth, and way beyond if you head in the tech direction.
Most people seem to forget that decent quality modern 1st stages can typically flow quite a bit more than 100SCFM, which is the equivalent of draining an AL80 in about 40 seconds. The tank valve flows slower than a good first stage, and ALL 2nd stages flow much less, often less than half, of the flow rates for 1st stages.
The conditions that would favor the S600 over the S555 are things like very high current where you might want to stiffen the breathing to help prevent a freeflow from the current pushing on the diaphragm. Another 'condition' might be if your reg is getting close to needing service and there's a slight freeflow from the seat wearing, you could simply tighten the adjustment. But, in the case of these regs, there's a seat saver which releases most of the pressure on the seat while the reg is not attached to a tank, and there's a plastic orifice which quite honestly does not perform as well as the older metal ones, but also probably does not wear the seat as much.
It's up to you, either one is fine. As I mentioned before, you'd probably notice more of a difference with the G250V.
Or you could do the right thing, forget this new 2nd stage bit, and get a D400. You'll spend less and get a better reg, but that's just my totally subjective opinion.