MN have a different (longer) negative spring contact that is designed for the Millennium setup. N lamps do with in the Millennium setups but SureFire don't always official suggest the Classic N lamps for Millennium Systems (especially WeaponLights)
Which lamp do I think it better?
The N2 beam looks better (very round beam)
The MN60 puts out more light in a larger beam - beam is subrounded usually though. Not a big deal compared to the MN11 or MN15 for example.
Of course, the N2 in an M3T over an MN60 in the M4 - I would take the M3T for compactness and easier to use (CombatGrip, no so bezel-end heavy)
The N2 in the 9PT compared to the MN60 in the 12PMT - If I'm using a 3" TurboHead, I may as well get the most light out from it as possible give the option of those two lamps; so I'd go for the 12PMT.
I love the MN16 beams. I have three SureFires with them in right now (two M3T's and a 9P+KT). The MN15 was whiter compared to the N2 but the MN15 beam shape was not how I like it.
I use the MN61 in my M4, M4Z and 12PMT. I find the MN61 gives me the best output/beamquality/runtime 'ratio' of the three, four-cell Lamp Assemblies.
The N2 lamp is pretty robust. I find when field-testing that straight filaments remain focused and last longer then curved filaments. This is a general observation based on the bulbs I have and the bulbs I've ruined.
I'm finding it difficult to pick between the N2 and MN60 because they are for different models. However, I do like the N2 beam from practically all TurboHeads I've ever tried it with (this is to do with the N2's filament I'm sure)
Al