MIM's have poly finish on the reissues (aka Classic Series), whereas the American Vintage reissues use a nitro-over-poly finish, as opposed to a full nitro finish on, say, a Custom Shop reissue or Thin Skin series.

Poly is just about bulletproof, which is why Fender now use it instead of nitro. Nitro, however, ages quite nicely over time and (if you buy into this) allows the wood to resonate more.

I'm shopping for an American Vintage '62 right now, but have also played the MIM reissues. They are quite nice. Necks can be a bit sticky, though.