At the risk of being over bearing, let me repeat that having used both, in my opinion buying a Mk 25 is a mistake now that the Mk 17 is available. The Mk 17 absolutely will not freeze flow on you in cold water while the Mk 25 has a bad habit of doing just that in water colder than about 45 degrees.
The 300 SCFM air flow of the Mk 25 is much greater thant the 177 SCFM delivered by the Mk 17, but it is also overkill and a 177 SCFM is more than enough for any technical trimix or deep air situation. The response rate of the Mk 17 is also excellent and it breathes as well as or better than my previous Mk 20's and Mk 25's with the same second stages. The Mk 17 is also fully sealed and this makes after dive maintenence a little easier as there is no ambient chamber to flush.
Based on my recent shopping experience and the various prices quoted to me, a MK 17cost me $50 to $70 less than a Mk 25 with the same second stages.
A not often advertised fact is that your LDS can also order any Scubapro second stage you want with the Mk 17. You lose the slight advantage of package pricing but a Mk 17 G250 HP can still be had for a retail full warranty price of $543 and from a "most bang for the buck" standpoint is the way to go.
In comparison a MK 17 S600 retailed for $598, although the cool thing was that for the same $598 as the MK 17 S600 I could get the pre-packaged Mk 17 X650. I own one and have tested it at depth and in 33 degree water with no issues with freeze ups. If I did not still own several very nice D400's it would be my second stage of choice. And all of the above prices were cheaper as the LDS offerred a 10% discount.
So basically I figured I could save money by going with the Mk 17 and get a better deal on what I regarded as a much better regulator for technical diving, cold water or contaminated water diving in the process.
There's nothing wrong with buying a Mk 25 as it is a well provena nd bullet proof design that offers excellent performance in most diving situations (the only exception being cold water diving). But be sure of your reasons for buying it and understand that you will be paying more money for a regulator that will not deliver any more useable performance than a Mk 17 and that it will be less reliable in cold water.
