Scubapro second stages can be confusing
The R series all use the same basic unbalanced poppet design. The R190 is the full size versiona dn the R290, R295, R380 and R390 are all compact versions and differ from each other in cosmetics and in the case of the current R295, in the deletion of the adjustable flow vave. All breathe the same which is too say ok out of the box, a lot better with a good tuning, but never quite as good as the balanced poppet G, S and X series second stages.
The G250HP and S600 are identical internally, the G250HP is just the full size version while the S600 is the compact model. The S555 is an S600 without the diver adjustable inhalation control. All use the same identical poppet assembly.
The X650 uses a shorter versions of the same poppet and is mounted in a case that allows delivery of the air straight from the air barrel into the mouthpiece. Its performance is nearly identical to the S600 with equally low inhalation effort, slightly smoother airflow, but with slightly worse case fault geometry so you end up adjusting it a little more.
Price wise the G250HP gives the most bang for the buck by far with performance as good as an S600 or X650 for a lot less money. The larger case accommodates a larger diaphragm that theoretically gives more mechanical advantage. The larger case is slightly heavier, but it displaces more water as well so underwater they feel exactly the same with perhaps a bit less bubble interference with the larger G250 Case and exhaust T.
I'd seriously consider getting a Mk 17 fist stage rather than a Mk 25. It offers 177 SCFm flow rate which, while a lot less than the Mk 25's 300 SCFM, is still more than adequate for any recreational or techical diving purpose. (300 SCFM is serious overkill and you will never use the extra performance any way.) Both first stages give identical performance at the second stage even at depths well in excess of recreational limits. The Mk 17 is lighter, more compact and fully sealed making it a much better travel reg and one that is easier to maintain.
The Mk 17 is packaged with the non adjustable but balanced S555 or with the balanced and fully adjustable X650. SP packages its regulators in such a manner to create beginner, intermediate and advanced regulator packages and tries to entice you to upgrade both first and second stages to gain additional features and the Mk 17, is currently marketed as SP's intermediate first stage - a position that badly underrepresents it's exceptional capabilities.
However, any SP dealer can special order any SP first stage with any SP second stage. It costs a few percent more to buy them separately, but because the S600 is a little less expensive than the X650, a special ordered Mk 17 S600 will cost the same as a Mk 17 X650 package. A Mk 17 G250HP package will cost a lot less than either of them as the G250HP second stage is significantly less expensive.
With regards to any of the regulators from other companies that you are considering, it is hard to go wrong buying an upper end regulator from any company -they all do pretty well. The difference comes in terms of service and support after the sale. Scubapro and Aqualung are companies that both have excellent records for supporting their discontinued models for decades and both also have very large dealer networks making service easy to obtain. If you have an Aqualung dealer nearby, consider a Aqualung Legent LX Supreme. It offers performance roughly comparable to the Mk 17 G250 or S600. From maintainence perspective, I much prefer to work on SP designs, but perfomance wise the Aqualung Legend LX is a good choice too.
