Someone please point out the specifics: wall thickness, the machining of the threads, shape of the threads, knurling, mating of the seams, finish, waterproofing, lens, QC, etc.
I can take a stab at that...
Ladies and gentlemen, in this corner, my Surefire G2 (original nitrolon head) and 6P (put together from parts, actually). And in this corner, a Solarforce L2M, L2P and assorted tinkertoy parts (bezels, tailcaps, heads, clips, lanyrad rings, etc.)
Wall Thickness: the Surefires are built for 16mm cells; the Solarforces both accept 18mm cells, so the walls are thinner. But not
thin; they feel plenty sturdy, and the l2M even has a screw joint in the middle to allow it to use either one 34mm-long cell or two (or a single 176670 or 18650). So, thinner walls, but greater flexibility in cells. You choose.
Shape of threads: the same. All four share the same head and tail threads, and hence same thread shape. (For some reason, the Solarforce heads use a slightly different thread for the bezels; seems odd, since they made sure to match the other threads, why not the bezel?) Thread are all smooth yet firm, even the nitrolon G2.
Knurling: SF G3 has a molded-in waffle pattern that's very easy to grip, but you can't really compare that to the others. The 6P is nicely knurled, grippy and well-cut. The Solarforce L2M's knurling is very shallow, worst grip of the lot; the L2P knurling, though, is similar to that of the 6P.
Fit & Finish: All four fit together very well. By design, all of them leave a gap between the body and tailcap, from .5mm to 3mm depending on how you mix and match tailcaps, to allow for a lanyard ring. The L2P is the only one with HA-II hard anodizing, and it's an even, satiny black; the L2M is only Type II anodizing, but it's even in color and consistent between all body parts. Surefire is famous for mis-matched anodizing colors, but mine matches very well, especially considering the parts came from different sources! The G2 isn't anodized, natch, but the color is consistent.
All four seem well waterproofed, with o-rings in all the right places; I haven't taken any of mine snorkling, but haven't and any leakage problems.
One thing you didn't ask, but I'm gonna answer anyhow: the switches. Both Surefires have simple twisty lock-out tailcaps, which I don't care for; both have been replaced with NetKidz' excellent forward-clicky replacement innards. The Solarforce lights are both reverse-clicky from the factory; the L2P has a longer tailcap with the switch recessed, allowing it to tailstand. Clicky feel on both is very good; I don't own a Surefire clicky to compare to, but they're as good as any reverse clicky I own.
The optional Solarforce forward-clicky tailcaps are another matter, though. The two models I have both use the same switch mechanism, which is very "mushy" (but quiet, unlike most clickies; the tailcaps themselves are VERY large, making the back end of the light seem bigger than the business end. I've bought some aftermarket forward clicky switch that are supposedly the same ones used in Fenix and JETbeam lights, and supposedly straight replacements for the Solarforce reverse clickies; I haven't gotten the soldering iron out yet, but from what I can see, they'll only replace the L2P's switch directly; the L2M will need a bit more work.
Another bonus item: the aftermarket. Solarforce sells a good range of accessories and replacement parts, offering a lot of flexibility in configuring your light to your needs; but the Surefire aftermarket is much bigger (albeit much pricier!). You could go crazy trying out every possible "LEGO" combination (and go broke trying to buy every piece that fits!)
So in conclusion: I can't really compare the G2 to the others in quality, due to it's unique construction, but it's a quality light. The L2M is a cut above the Chinafire lights in fit and finish, with an excellent switch to boot, but not quite in the same class as the Surefire. The 6P is admittedly Surefire's entry-level light, but well made; it's only real weakness is the relatively delicate Type II anodizing (and the LOTC, but that's more a matter of taste). IMNSHO, the Solarforce L2P is the best light of the three, with fit and construction nearly as good as the 6P, but with the advantage of HA-III hard anodizing and an excellent switch in a tailstandable tailcap.
That concludes today's lesson. Any questions? Anybody? In the back? Beuller?