Frank, the previous posters covered most of the bases.
Let me add a few remarks, if I may - I feel I have a right, since I own both 🙂
Viewfinder/framelines: depends entirely on your lens preferences. If you're looking for a "universal" body, then M6 is probably a better choice, meaning it's less restricting. It's not very precise for longer lenses, but at least you don't need an external finder or "goggled" lenses, as is the case with M3 and wider lenses. It also depends on the VF magnification - most M6 cameras come with the default .72 magnification (like the M2). If you really want to use longer lenses, and feel you have problems focusing, you can always get the "focusing magnifier" thingy (not cheap, though...) OTOH, I've never had problems focusing a 135 Hektor on my M2 (same magnification as M6). But, I usually shoot static objects with a 135 😉
Loading: if you never loaded either of them, this is a non-issue, since both might be a bit difficult at first. I've learned the "Leica masochistic dance" or the "three-handed rap" (read: loading a Leica) on my M2. Got it right the first time, and never had any problems. It's a lot less fiddly than a Kiev 4A, for example.... M6 is relatively new to me, and I must admit that I load both my M2 and M3 a lot faster than the M6 - however strange it might seem... So, I guess this is again relative. I'll probably get used to M6 soon... 🙂
Rewinding film: unless you're in a big hurry, a couple of seconds more for the "old style" knob on the M3 isn't really that important. But, it's definitely slower than M6, and might be a problem with gloves (something you should take into consideration, I guess). But, since you're in Canada, and will probably also be shooting in winter, this might not be that bad - at least you won't be in danger of building up static electricity when rewinding too fast 😉
The only issue, I think, is the actual condition of the camera. If you can test it prior to buying, it would be a big plus. Both cameras might have problems.
To sum things up, I'd NEVER buy a Leica (or anything that expensive) on a camera show, without testing it first and (possibly) having it checked. Yes, you can handle it in person, but are you sure you can find every defect in those couple of minutes you have it in your hands, without developing a roll of film you have shot using every possible speed setting? What about shutter capping? Light leaks? Flash sync? The camera might look wonderful, but you might get mighty disappointed when you get your first roll back.
I can only offer my recent experience - the M6 I bought from a photo.net member, who said the camera was CLA'd by Sherry two months ago (a BIG plus!). Said it had some intermittent light leaks.... fixed by Sherry. The person who sold it to me actually used it (a plus again). Also, he provided a 10-day money back guarantee - so it was sent to Brian for testing - and the camera passed all Brian's tests with flying colors (thanks, Brian!).
What I'm trying to say is that I'd rather buy a Leica "sight unseen" from a reputable RFF or photo.net member, than on a camera show. I've bought cameras on shows before, and I know how it is: usually you're too excited about it, and tend to overlook small (?) defects, in fear that someone else might snatch it 🙂
Sorry if my ramblings are more confusing than helping... I just wanted to say that both are good choices - it all depends on the actual condition of the camera.
Finally, looking back, it it were my first Leica, I'd probably go with the M6. The difference in price is not that great, and you get a metered camera, AND a camera that's about 30 years younger. The prices for well preserved, late model and fully working M3s are very close to M6s.
Denis
