I lean heavily towards the AFCK, but wouldn't fault someone for buying either. I've handled and used the AFCK hard, but only handled the LLC.
On the handle side, Benchmade's ergonomics are usually far superior to Microtech's, but this is subjective, hands vary. The LLC is better than most Microtechs, but the AFCK has just incredible ergonomics that can't be beat -- yes, that's subjective, but everyone I've handed an AFCK to loved it, not as true with the LLC. Because of the LLC's thickness and weight, and the AFCK's length, neither are the most easily-carryable knives in the world.
Microtech's liner locks have, in my experience, generally been mediocre. Benchmade's have been superior. However, again, the LLC seems particularly well done, so this might not be as much of an advantage for Benchmade as it usually is.
Blade-wise, both have very functional blade shapes. The Spyderco-style opening hole (as on the AFCK) blows away any other opening system, in my opinion. The LLC will be much much sharper than the AFCK out of the box. Microtech has a better reputation for heat treat than Benchmade, I personally think it's overblown a bit, but Microtech may very well have an edge here. However, the M-2 AFCK will beat Microtech's 154-CM, even if the Microtech 154-CM beats Benchmade's ATS-34.
Microtech gets all the fit & finish details -- grind lines,action, handle shaping, etc. -- better than the vast majority of production companies. The AFCK is still compentent here.
For a pure heavy-use using knife, I'd go with M-2 AFCK.
Joe