[dpattinson]

If I were buying a 'modern' film leica right now, then I'd almost certainly choose a used M6 (I already have a M7 - so for me that question is moot). The fact that you're considering the MP means you think you can do without auto-exposure, and your use of a M3 proves you can handle that lack. If you intend to buy an M8/9 later on - then the M6 will cost a heck of a lot less as a stopgap in the meantime.
In any case - there really is very little practical reason to be buying a 'new' leica film camera right now, used ones will be fine and will save you quite a bit of cash.

(a bit of heresy - you should also take a look at the hexar rf. It's less than half the price of a M7, has 2 stops faster shutter (think Tri-x at f2), and auto exposure with a 'getting used to the M8' autowind into the bargain).