If you take the beam (output/runtime) as one issue, and the bezel as the other...
The beam from the KL3 (current) is very different from the beam of the P60L. You may find that you need/prefer a tighter beam with less spill, or not.
The output is quite different in terms of lumen output with the P60L output obviously greater. This will reduce the runtime of the P60L compared to the KL3.
It would be neat to see a runtime/output chart for each at this point to compare them.
The P60L is an LED Module "Lamp Assembly" and directly replaces P60/P90 Lamp Assemblies in all existing products.
The KL3 is a replacement bezel - it is larger in diamater, longer and heavier than the standard (6P) or even Z32 Shock Isolated Bezel (although more similar to the old style M2 bezel in size and heft)
I feel the P60L has far more appeal to a far wider range of users and applications compared to the KL3 bezel. Unless SureFire update the KL3 I suspect it will be discontinued (this is only a guess, I have no evidence at all to support it)
I hope this helps?