Surefire would be most expensive but used by military all the time. Streamlight is cheaper, used by law enforcement too (I'm sure they use surefire also). ElZetta, Modlite, cloudlight I've heard good things about but in comparison to those other two, they may be relatively new on the block or don't have as large a customer base.
Surefire has a reputation of good customer service, and their products seem to last so long that I've seen guys trying to sell their old halogen rifle lights after having them so long and wanting to move on. Some can be retrofitted to use LED bulbs.
I have a couple Streamlights and have used their customer service on a rifle light I buggered up.... disassembled and couldn't get a gasket back on. I contacted them on their online form, followed instructions online, sent it in to them, and they sent me a new one after a few weeks. All i really wanted was help getting this asymmetrical gasket in the right place.
I have a Streamlight TLR-1 HL, which would be a competitor to the Surefire X300U. I got a police trade in for around $65, a new one would be around $100 or so, but even at that price i think it's worth it.
Olight has a design that was "inspired" by the surefire and streamlight design. The PL-2 Valkyrie, or the BALDR (with green laser), side by side CR123A design. The problem with Olight's PL-2 Valkyrie is that yes, it puts out something like 1000 lumens but then ramps down the output within 3-5 minutes due to the heat. If you see yourself using momentary flashes, may not be a big deal but if you might need it on for longer, may want something more consistent. But then again, if you are going to use this defensively you may want something sturdier. The PL-Valkyrie's switch is digitally controlled, so it if you wanted a quick momentary flash, you can't really do that like a manual switch can. A really quick click of the button would turn the light on and leave it on. A longer press then release is it's momentary setting. I bought a couple for project guns, not serious use. Comparing the PL-2 Valkyrie to the TLR-1 HL, the aluminum in the Streamlight's body is thicker and has a thick gasket that is compressed at the battery cover rather than an O-ring to keep stuff out.
Olight makes a new version the PL Valkyrie, the Valkyrie Pro - which you can choose a lower output that probably doesn't need to ramp down BUT there is a built in rechargeable battery that can't be removed, can't be replaced. Probably lithium ion. So to me that means there is a finite lifespan on it. I know my computer / phone batteries start to diminish in capacity in 4-5 years but are usable longer. You probably won't be discharging a weapon light frequently, it would likely last longer than 5 years assuming no other mechanical problem.
I have a smaller Olight pistol light, the PL-2 mini, also for a fun project gun. It's bright, but rechargeable only battery means it's going to give up the ghost in time and it's not user replaceable.
The TLR-1 and Surefire X300u (I think they may be up to x400 now) designs are kind of older, the two side by side CR123a. It makes for a wide profile. If that doesn't bother you, the wide lens makes for a great beam. I think streamlight is up to the TLR-8 now which seems more trim.
If you're anticipating using it indoors mostly, some would say don't go too bright on the lumens. When you get to 1000 + lumens in a white painted hallway, there's a good bit of flash back. On the other hand, I've heard the argument that super bright will make an assailant flinch rather than just signal where you are.