I own an M6 (classic) and have some light experience of M4-Ps and, honestly, other than the meter there really isn't much between them. The M6 is a SLIGHTLY cleaner design in my view (no exposed screw-heads on the body, that kind of thing), and the viewfinder on my example is certainly brighter than the heavily used and substantially older M4-P I'm going by, but seriously there's not much in it. It really would come down to whether you need the meter or not (I felt I did, but I'm currently looking at a meterless body for backup).
More to the point would be the difference between the M6 and the M6-TTL. Bear in mind that the TTL's shutter dial turns in the opposite direction to your M2's, so if you plan on running both bodies as a kit, I'd stick with the classic to prevent confusion. Some people report being able to switch between old-style and modern-style shutter dials without difficulty, but I know it'd drive me very mad very quickly.
FYI the M6 classic has one fewer LED in the display (which I prefer), is about 40% easier on batteries, is a couple of millimeters shorter than the TTL and (in the UK at least) tends to be cheaper.