I couple of notes from a guy who (non-professionally) owns a bunch of SP regs and has had 'em all apart at least once.
SP has, indeed, "cheapened" and "complexitized" (is that a word?) their regs over the last 10 years or so. The most striking examples of this are the Mk10 and Mk25, side-by-side.
The Mk10 is a very simple design. They could have named the reg for its parts count! Seriously. And yes, that includes O-rings.
The CLAIM is that the Mk25 is a performance improvement (actually, the Mk20 is, and the 25 has the external adjustment.) They've also diddled with the cold-water anti-freeze protection over the last 10 years or so.
But what I can tell you, having all of them side-by-side, and having tested and actually USED all of them, is this:
1. The Mk10 was a very simple design, and works really well. It can be overhauled by a monkey in his sleep; I can literally strip and rebuild one in 10 minutes, not including cleaning time (which of course is variable depending on how gunked up it is.)
2. The Mk10+ has the exact same HP seat design as the 20/25 - in fact, seats for the 20/25 will work in the 10+ and vice-versa! That, by the way, is the only difference between the 10 and 10+. They are otherwise internally identical (only the piston and seat change.)
3. The IP drop under demand is imperceptably different between the Mk10+ and 20/25. There is a difference in performance between the older knife-edge piston design on the Mk10, and the "concave seat/flat-piston-edge" design in the 10+/20/25. Its not major, but its there (the difference being that a Mk10 might show 10-15 psi IP drop during a strong inhalation or purge, while the 10+/20/25 might show only 7-10psi.) That translates into more responsive air delivery, especially with non-balanced second stages. However, paradoxially, the 10+ (and presumably the 20 and 25) are more sensitive to issues with the piston itself.
4. As such, the "newer design seat/piston" IS an improvement in real performance.
HOWEVER, the 20/25 introduced these damnable plastic bushings that "capture" the HP O-ring, rather than the tried-and-true groove in the actual regulator body. The result of this is that the newer regs have LOOSER internal tolerances (plastic simply isn't as stable as metal), the plastic pieces WEAR (I've actually had one of these bushings CRACK in normal use, resulting in a nice leak), they could fail catastrophically (which would lead to a HUGE HP leak; if it happened underwater you'd be extremely unhappy), they are on the "must replace annual" list, which now means that you now are tied even more than usual to SP for parts and it ALSO has driven a requirement for special 90-duro O-rings at that location - 70s will LEAK BADLY in the 20/25s, while they work just fine (EPR please, no Buna!) in the Mk10/10+.
If/when Rofales starts paying attention to service issues in their reviews, all of this would get WIDE publicity. And the simple fact of the matter is that my cynical side says that they did all this garbage in the 20/25 simply to be able to file another patent - not for real innovation, but as a means of keeping others away from their "great design". In other words, its all marketing hype and no substance.
Then you have the "borg turret" on the new Mk25s. I just saw those recently. It appears to be yet another attempt at making a cold-water reg out of a piston design without using an environmental kit. The entire CONCEPT is a crock in my opinion, but heh, marketing reigns supreme, right? The proof that its a crock, by the way, is that SP has been doing this "thermal insulating system" garbage in one form or another for more than TEN YEARS and is STILL changing it damn near every year in an attempt to actually make it work! The obvious REAL solution is to just pack the silly thing with grease, but that's both messy and (if you use Christolube) expensive, and makes overhauls take longer since youi have to clean all the old crap out of there when you're doing the overhaul. THAT makes it unpopular as an option (as it should!)
IMHO the truth is that piston regs simply shouldn't be used in water under 50F - if you're going to dive cold water, do it with a sealed diaphram first.
The same appears to be true on the S600. There are a lot of plastic bits in there (including the air tube) that are not in the older G250. Yet the 250 breathes almost identically to the S600 - and its a 10-year+ old design! Further, the 250 is a very simple externally-adjustable barrel poppet, while the S600 has a lot of CHEAP, SOFT PLASTIC parts (not even hard, good-quality plastic), particularly in the adjuster and poppet tension setting arena.
The ONE place where the S600 wins is that it unloads the poppet when the reg is not pressurized. This is a REAL advantage, in that it extends seat life almost indefinitely. So there is a real advantage there. But that poppet seat is a $0.50 part, in all honesty (even though you have to buy a parts kit to get it!), if its worth that much.
On the G250 you can get nearly the same results by popping off the metal clip that restricts outward movement of the poppet adjuster and backing it out until it unloads when the reg is not in use. That doesn't change the reg's adjustments, and can be reversed when the reg comes out of storage in about 30 seconds.
