There is also a Canon 28mm f2.8 lens - smaller than the 1.8 version.
Personally, i'd just use the Canon lenses and not bother with adapting other brands. I've tried that route, but always came away frustrated and without any tangible benefits. For one thing, the dSLR viewing screen is not ideal for manual focusing. You can get an alternate screen, which is supposed to be optimized for that purpose, but it's not a very significant upgrade. Secondly, i've had issues with focus consistency when using manual focus lenses on both the original 5D and 5DMkII. With both standard and focus confirmation adapters, i'll see it in focus in the viewfinder, but the LCD review shows it (somehow/mysteriously) out of focus. I've never been able to explain that, and i've seen the same comments from others online. Doesn't matter, either, what brand of lens i'm using. I've adapted Nikon, Contax, Pentax, and Leica-R.
The hassles, also, of shooting with stop-down metering are also notable. I suppose that, and maybe the focus/viewscreen issues, are moot, though, if you're going to scale focus and shoot from the hip, but are you really going to do that 100% of the time?
A 5D and one of those smaller non-L primes is not heavy. An M7 with any non-pancakey lens feels denser. I think, also, as pointed out above, that most people are so accustomed to seeing dSLRs that they are now quite inconspicuous. I don't think anyone is going to 'blend in' to the background and become 'invisible' - Leica or no Leica. A Leica user is bound to be perceived as a person shooting a big compact digicam. No more or less common than an SLR.
I've never used either of the Canon 28s. I used to have the 35/2, but now have the 35L. The 35L is a fantastic lens. But, it's big. The 35/2 i compared to the Leica-R 35/2 and found them to be even. Bokeh is pretty good. The L, though is better on all accounts. The 50/1.4 is a really nice lens. I always thought the bokeh was excellent, until the Sigma 50/1.4 came out. The Sigma is also pretty big, but bokeh-wise, it's amazing. I wouldn't advise getting one, though, unless you get the 5DMkII (or any of the other, newer Canons that have in-camera AF micro focus adjustments). Same with the Canon 50L - another huge lens. I have this now. Again, great bokeh (still, i think the Sigma's is better), but it requires the micro adjustment feature.
I haven't tried the zeiss for EOS line. I wanted to buy the set, though, as i've loved the Zeiss lenses on the Contax Aria/RX, G2, Contax N1, and Hasselblad/Rolleiflex, etc. But, i've hated the bokeh in just about every relevant Zeiss ZF (for Nikon) 50/1.4 shot i've seen. I've read it's the same formula for Canon. The 85/1.4, though, looks very nice. But, then, there's the Canon 85/1.2L, which still beats it....
Here's the thing, though: If you were impressed by "all the full frame comparison stuff," you could still stick with Leica, if you already have M-mount glass. Wouldn't an M8 with really fast CV glass like the 35/1.2 or 50/1.1 give you the same kind of DOF as a full frame 5D with 35/2 or 50/1.4? Not that i'm advocating the M8. I never bought one, as i also was only interested in full frame.... But, that's a thought, if the DOF thing is the primary concern. I would suggest, though, that if you're scale focusing and shooting street stuff, that's not really where you're going to see the benefits of full frame (5D/M9 versus M8).
