The DPMS 18" 1:10 bbl uses a midlength gas system.
The 20 & 24" systems uses a 12" rifle length gas system.

I am wondering if the 18" would be better served with the RL gas system?

Along the same line of thought, their 16" bbls are carbine gas systems. Would the 16" be better with a ML?

Yes I am wondering if I should get what they offer, or buy longer and cut to the length I want

The idea would be for a 16"-18" 308 SPR that would work well with a wide variety of ammo (110 through 175g) and with/without a suppressor

Anyone ever done a piston conversion on a 308?

FWIW, Armalite's 16" .308 is on a midlength gas system, and their 20" and 24", on a rifle length. Reliability with a wide variety of ammo is not there out of the box, as much of what's out there isn't hot enough to adequately cycle the action, but they can be made reliable (but will probably be overgassed with hotter loads). With/without a suppressor will probably require an adjustable or at least couple-position gas block.

Isn't Larue's OSR piston driven? Or am I on crack?

What does the R25 have?

20" bbl with 12" rifle length gas system