Like Funtastic said, the CSLNM1 osram is more intense, which makes it appear brighter (at the hotspot in the middle of the beam), but the spill of the CULPM1 is brighter allowing you to see more around the hotspot and theoretically lets you see more field of vision in front of you, not just really far down range / in one small concentrated spot near you.
The 3x21a or 4x18a with (3) osrams would be awesome, and is a modification i am acutally going to do to a different 4 x emitter light, i think it would be cool. It would throw further than the sft40 3x21a, but in my opinion the beam color and quality may not be as good. it is hard to tell with those multi reflector lights until you have them in hand how good the Beam quality is with any given LED.
This light will get very hot, hotter than the single sbt90 light or at least just as hot ( i think the 3x21 is listed at running at 17.8 amps, and Simon said his sbt90.2 driver is an 18amp driver) so they may be very similar, but the 3x21a i dont think would run noticeably less hot. Also, the battery life ( if both are same configuration) would probably be similar because their both running drivers with similar amp output, so one shouldn't be much better than the other here either. The 3x21a is putting out a lot of output, it will be noticeably brighter than the sbt90, and have larger spill, but the 3x21a will not throw as far (a triple reflector light throws around 20% less than the equivalent led in a single reflector light, i think i read somewhere form U/TomE). so the 3x21a could fill the gap between the single osrams or sft40 light and the sbt90 light, but in reality in some ways it surpasses the sbt90 light.
I think you just need to remian focused on what your goal is for this light -
portable?
bright "spill"?
long range ?
long range and bright "spill"?
1 battery or more than 1 battery?
internal charging?
brings me back to this - if you dont need charging then get the C8+ with cslnm1 (price vs performance = this is a winner, as other shave said). if you do want charging get the 3x21a or 4x18a (either triple LED or sbt90, respectively - they will be similar enough in output, throw, and awesome performers, with differences i mentioned above). The L21a is awesome, but heavy and the tradeoff in weight probably isn't worth the loss of portability you get with the C8+, if portability is important to you. this is where the L21b may be a good compromise - lighter than L21a, better throw than C8+, but with performance close to L21a and weight closer to C8+ than to L21a. its just kidn've.. blah looking. lol