Depends on the set-up, but I would probably stick with 6v (2 cell) lights.
OP: For a 16" FSB carbine w/ MOE hand guards:
I'd probably go with a Inforce WMLx, or a Streamlight HL-X in a ring-mount.
The latter is a "dual fuel" light, so you can also run 18650 rechargeables, as well as CR123 primaries.
For railed, non-FSB rifles: I prefer to have the light on the top rail, in front of any BUIS you are running.
In that application, I'd probably still go with an Inforce WMLx (gen 2) if you have the rail space / length. If rail space is limited (SBR, etc.), and/or you want to maintain a longer sight radius for your BUIS, I'd go with a Streamlight TLR-1 HL.
Either gives you a 6v high-output light with both momentary and constant-on functionality. And both give you ambi activation capability, while eliminating the need for any additional pressure switches/wires, etc.
The Streamlights tend to have a little better beam (TIR optics) and color temp / tint.
YMMV.