Marketing people really suck. (nothing personal intended if any readers actually do that for a living.)
In reality, the poppet assemblies on both are nearly identical. The poppet body on the X650 is a little shorter, otherwise it is the same S-Wing balanced poppet, seat and orifice design used in the S600, S550, G500, G250HP and upgraded G250, G200B, and Balanced Adjustable second stages.
The X650 and Legend actually use a similar "flow vane" adjustment in that both are different from the moveable flow vane used in the S600, S550, G250 HP, R390, etc. In the Legend, the air is directed by a deflector coaxially mounted on the air barrel and is essentially bounced off a fixed flow vane in the mouth piece. The X650 also uses a coaxially mounted deflector, but due to the design it shoots the air straight into the mouthpiece. This does give the X650 one thing in common with the D400 in that the airflow itself is very direct and very smooth compared to the much more turbulent flow that you get with other designs.
For all practical purposes the G250HP and S600 are identical, except for the size of the case, and the G250HP, S600 and X650 are all capable of the same excellent level of performance, with the exception of the G250HP and X650 being better choices for cold water.
The Legend first stage is essentially a tried and true Conshelf first stage adapted to have the air enter from the side. It also incorporates a diaphragm seal like the Conshelf Supreme and uses a plastic pad and stem arrangement to transfer ambient pressure to the diaphragm. It's a solid design, but is none the less a 30 year old design that is a little dated and offers less flow rate than a Mk 17.
The advice given above to avoid the "new" Mk 17 until the design matures a bit is normally sound advice. But given that the Mk 17 is an evolutionary derivative of the already proven Mk 16 and given that it has been in development at SP for the last 2-3 years (that I know of) and has been in service outside the US for the last year or so, I don't have my normal qualms about buying a Mk 17 now. In fact I bought two of them.
Despite how they are cataloged, an SP dealer can sell any first stage with any second stage. It's those sucky marketing types again who advertise specific packages in order to create "entry level", "intermediate" and "advanced" regulator packages with the goal of selling the average diver at least three regs in their career.
In reality, it is easy to order a Mk 17 with a G250HP or whatever other second stage you want. The dealer just has to fax the order in and it will arrrive from SP in a week to 10 days. You lose the few percent discount you get with their cataloged packages, but in this case you will still save money if you go with the less expensive (but equal perfoming) G250 HP.
