I was going one direction with my weaponlight on a Centurion Arms CMR 9.5" handguard with CMR Scout Mount, but there are clearance issues with my original mounting plan.

So since I am going to use the standard CMR rail mount, I have 2 options that are close to the same price range.  The Surefire X300U or the SF M300B.  A pressure switch will be added so the cost of the pressure switch is also being considered.

X300U = 500 lumens with 1.8 hr run time.  2x CR123.  Pressure switch costs ~$180.
M300B = 200 Lumens with 1.3 run time.  1x CR123.  Pressure switch costs ~$100.

Both lights cost about $200-250 used/new.

It is being mounted on a 10.5" SBR. So the rifle was put together for close range stuff, but I wouldn't mind if it was able to reach out further with the light.

One concern is that the 500 lumens is too bright for indoor work.  Another question would be which has better spill to illuminate a room?  Any recommendations or reasons why you would choose one vs. the other?

X300 is going to produce better spill and IMO not too bright for indoor work.

That being said I run M300s on my SBR CMRs.

Check this out as well.

I might just see what pops up on the EE and go with whatever deal I see first as they both seem like good options.  The X300 is actually lighter than the M300B, which makes it look more attractive.

I've personally broken an X300 mounted at 12 o'clock from a relatively light knock. I use them on pistols only now.

Shine an X300U against a light colored wall in the dark at close range sometime, it's a little much for me. YMMV of course.

That was one concern, was the reflection of the light on light walls.  I think I am leaning towards the M300B since it already comes with a pressure switch until I can pick up the SR07 switch.

If you intend to use a pressure switch, then the m300 is the way to go. As a long time Surefire fanboy, I have to say the pressure switch tailcap is one of the worse designs I've ever seen on a light of decent quality. I broke mine twice, doing relatively low intensity stuff.

Is that with the ST07 or SR07 pressure switch?  Or both?

I ended up buying the M300B last night.

The X300U will have more spill and is not too bright for indoors, but I would not recommend it for a rifle unless it's mounted on the top rail and protected from impact by a fixed front sight behind it.