If you're going to be using MF Leica lenses on your future camera then I'm guessing you will use the setup for 'slower' work, the kind that has less action, is more calculated and methodical. If I am wrong, forgive my ignorance. However that's the way I currently work with a 1Ds and Leica lenses.
The AF and FPS of the 50D would hardly mean anything to you in this instance, and as Adam mentions the larger VF makes accurate focusing easier. A number of people who have tried the MF lenses on a 1.6X crop camera have issues with getting accurate focus. And to be fair, I sometimes have issues with my 1Ds as well with wide angle lenses and faraway subjects.
It seems sensible to hyperfocus in such an instance but I find the distance to be inaccurate. Furthermore the Leica lenses seem to perform best wide open and just stopped down a couple of stops. When you start pushing a such lenses to f/11 and beyond the quality usually drops. That's what I found of a Summicron 35/2 anyway, where f/4 gives screaming performance, f/8 shows stagnation of IQ and f/11 actually loses sharpness in the borders.
However beware that the 5D is said to have a larger mirror than most other Canon FF cameras, which necessitates shaving of the mirror in some instances. There's a useful reference here for what fits and what does not:
http://www.pebbleplace.com/Personal/Leica_db.html
Not to steer you any other way, but if I were into alternative glass and doing all this slow, methodical stuff, I'd rather use a 1Ds Classic than a 5D.
There's a big debate going on as to which format is more taxing to a lens...is it a FF sensor or a 1.6X crop sensor?
One school of thought is that FF sensors utilize more of the image circle in a lens and hence has the tendency to reveal flaws in a lens that would exist towards the periphery, rather than the center. This includes issues like CA and unsharpness.
But the other school of thought is that 1.6X crop sensors have a much higher pixel density, hence requiring more lens resolution. Each of the tiny pixels needs to be enlarged a lot more in producing the final image. This in part may explain why compact digicams cannot give a result equal to that of a DSLR of a similar age and technology, and why medium format lenses are said to actually have less resolution than a 35mm lens. But when you look at a print from a medium format camera, the images are stunning. Why? It was enlarged less, despite whatever size it was printed to.
Ultimately the key thing to remember is what the crop factor does to the applications of your lenses. A 35mm lens is 35mm on a FF camera. It is usable for wide angle shots. Put the same on a crop camera and you get 56mm. Now it's more of a standard lens than a moderate wide angle. IMHO, this is the most important principle to remember when considering crop vs FF when considering alternative glass. For some, the crop factor helps, but for others it's just a hindrance. I will not buy a Leica 35mm lens just to use it at 56mm equivalent field of view on a crop camera...but that's just me.
