I second the recommendation of the IIB as a good choice for a user. It isn't as advanced as the last Barnack-like Canon, the IVSB2, but it is the first major export product of Canon. They were sold in considerable numbers in the Gov't PX system, and by distributors in a number of countries. It also incorporated the coupled RF/VF window, which Leica didn't seem to consider important until a bit later. 😀
As has been said before, their isn't a lot to differentiate the various models, but that is also true of the later Leica models, until the IIIf/g with the ST and/or extra viewing glass. I have a IIIa and a IIIf RDST, and am content with my Barnack-type Leica models. I have several more Canon models, but that is partly collecting interest, and partly financial, since they are considerably cheaper in most cases.
Many of these cameras are now in need of a competent CLA, and possibly shutter repair/replacement. There are a number of good repair options, since some of the Leica people will also work on Canon. That factor should be taken into consideration, unless you want just a shelf queen. The later M3-ish type Canons are excellent picture taking machines, and are as different from the early models as the M-series is from the Barnack Leicas. Either will deliver an excellent negative, but there is a convenience factor or a nostalgia factor that comes into play. I like both, for different reasons, and indulge myself for the pleasure of owning and using them. Gearheads of the World.. Rejoice, for prices are nice right now!
Harry