The Canon 7 is the best deal in Canon rangefinders in my opinion. It has a decent finder with nice, bright frames, covering the 35mm, 50mm, 85mm (also useful for 90mm), 100mm, and 135mm. While you won't be able to use wider lenses due to the lack of an accessory shoe, these framelines cover most of what you'll want an LTM camera to do, and you won't have to worry about carrying around accessory finders. When the Canon 7 bodies can be purchased for less then $200 in nice, working condition (less than a Leica 35mm accessory finder!), you can't really go wrong.
I don't like cameras that bunch up the framelines... especially three at once. The simple Canon 7 finder layout is great in this regard. I've never really felt the Canon 7 was a large camera, either. It's really a nice camera to use, if not to collect.
Like I usually say, you'll need to pick what you think YOU will like. As you can see, there are a lot of opinions around here!
In regards to an M2/M3 versus a Canon P/7, I never once felt the Canon 7 was a lesser camera than an M2 or M3. The M2/M3 have brighter finders with nicer rangefinder patches, but the Canon 7 had some tricks of its own (rewind lever, additional framelines built-in, easier film loading). The only reason I sold my Canon 7 was due to the lack of an M-mount.
