hamsiss:
But can you please explain the difference of S600 and X650 which is your choice and why?
I lke them both about the same, they are just different from each other and it really becomes a matter of personal taste and styling.
The X650 uses a case that is similar in shape to the D400, but does not incorporate the D400's coaxial exhaust valve and diaphragm so it really misses any major advantage from the case design. The poppet and air barrel location does however allow air to pass straight into the mouthpiece and the flow is a bit smoother than the S600. The X650 has also proven to be reasonably reliable in cold water.
On the negative side, you end up having to de-tune the X650 a bit to prevent freeflows in a face down position (since it does not have a coaxial exhaust valve and diaphragm). It's reputation has also taken a hit with a recall due to a design flaw right after it was introduced and with a very small recall of one batch of X650's that were made with the wrong mix of plastic used in the case. The recall do not really reflect on the design as the design issue has been fixed and the mix problem could have occurred with any model, but there are some things people don't forget or forgive. I use an X650 on my full face mask where it does a superb job (my lightly tuned D400's resonate and are not well suited to FFM applications)
The S600 is a solid design, although it tends to be associated with freeze flows when used with the Mk 25 in very cold water where some divers have observed very slight freeflows with the S600 that add to the heat transfer load on the Mk 25. That however would not be an issue when paired with the MK 17.
The G250HP is the same as an S600 except it uses a full size case and diaphragm and does not have the same association with freeze flows as the S600 (perhaps the larger case provides marginally better heat transfer?) It is however significantly cheaper than the S600 despite the fact that they offer identical performance. People seem to value small second stages, despite the lack of any real advantage (jaw fatigue is more effectively addressed through appropriate hose lenght and a good mouthpiece design and the larger second stage offsets it's greater weight with greater displacement underwater since much of the extra volume is extra air space) and a disadvantage due to the lower mechanical force availabel due to a smaller diaphragm. Since people will pay more for them, SP charges more, even though the production cost is no greater than for the G250HP.
My preference of currently available second stages in order then would be:
G250HP
S600
X650
If you add out of production second stages as well, my preference would be in order:
D400 (There still a few new ones left as old stock as they were dc'd in 2003.
G250 (the older metal air barreled version)
Balanced Adjustable (the all metal cased precurser of the G250)
G250HP
S600
X650
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