If you go with a 5DC, check to see if the mirror recall repair has been done on it, or plan to send it to Canon for the free repair. You can tell by looking at the mirror. If there are little clips visible near the corners of the mirror, then the fix has been done. If there are no clips, it hasn't been done. You might be able to search here and find some photos of the mirror before and after the fix.
Your usage is not the same as my usage. I held off buying 5DC because it's such a dust magnet and doesn't have the sensor cleaning found on the 5DII and your 50D. I often have to shoot in very dusty conditions (bad enough that I went to multiple crop cameras to avoid changing lenses as much as possible). The self cleaning system is really helpful on 5DII!
I think the camera's shutter is rated for 100,000 clicks. This is no guarantee that it will reach that goal... just Canon's prediction for life expectancy. The 5DII is the same, as are most of the other current and recent models. Canon rates the 7D at 150,000 and the more recent 1D series at 300,000, for comparison.
5DII saw virtually no improvement of its AF system, compared to 5DC.
I do use and like AF Microadjust, which allows fine tuning lens focus for up to 20 or 25 specific lenses. Your 50D has it. Both 7D and 5DII do, too. 60D and 5DC do not.
The new LP-E6 batteries used by the 5DII (and 7D, 60D) are quite good. They give nearly twice as many clicks. Of course, the older BP511A that your 50D and the 5DC use are more widely cloned and available inexpensively. I've bought generics as cheap as $6 or $8 apiece off eBay, that work just fine. Some third party LP-E6 are now available at lower prices, too, but be careful... some require their own charger, can't be recharged in the Canon charger.
I do like using a full frame camera in combination with an APS-C/1.6X crop sensor model. This really leverages the usefulness of my lens kit. I have no desire to get an APS-H/1.3X model.
However, I gotta say, my second camera is identical to my primary (currently a pair of 7D, but in the past it's been a pair of 30D or up to three 50D). My third camera is now full frame. However, weddings are way, way down my list of jobs. So my needs are different from yours! Many weddign photogs prefer to use full frame.
For fast, high volume editing, I use LIghtroom 3. For final image finishing, I use PS CS5.
High ISO is a very subjective thing... what I find acceptible you might not, or vice versa. With my 50D I hesitated using them above ISO 1600 and prefered to stay under that as much as possible. With 7D I'll shoot at ISO 3200. With 5DII I'll use ISO 6400 without too much concern. In part this is the newer cameras' capabilities, but partially too it's the most recent versions of Lightroom and Photoshop offer much improved noise handling. In the past, altho it was slower I used DPP for better noise control with high ISO RAW conversions, and then handed off the image to Photoshop for final work.