Have C645, Hasselblad 503CW and 203FE that I use on a regular basis.
Hasselblad has a wider range of lenses to select from. However, all of them can be adapted to the C645 with any number of adapters, but must be used with stopped down metering and shooting ... not good if you like to shoot at f/8 ; -) (I also use the Hasselblad lenses on a Canon 1DMKII digital body via an adapter, but tend to use them with wide open apertures).
None of the C645 lenses can be used on any other camera because they are electronically controlled apertures so they could not be stopped down at all even IF there were adapters, which there are not (all the Hasselblad lenses are mechanical, so they can be stopped down).
The C645 offers auto-focus lenses which the 500 and 200 series Hasselblads do not. It is a focal plane shutter camera that has a higher top shutter speed than the Hasselblad 200 series (1/4000th verses 1/2000th).
Hasselblad C series lenses are leaf shutter type and provide flash sync at all shutter speeds to 1/500th which is good for stopping action better, and for use with fill flash in bright conditions. The 200 series focal plane cameras sync up to 1/90th and the Contax syncs to 1/125th, which limits their use in certain conditions and with certain lenses.
Quality of the glass is fairly comparable. There are shining ex&les of outstanding performance in each system. I'd give the edge to Hasselblad here because there are more spectacular performers than in the C645 system.
Hasselblad 500 series is the most versatile MF system available, and virtually all digital backs fit it from the very first backs offered, to the latest ones just announced. Most also can be fitted to the C645, but we'll have to see if that continues as we move into the future because Kyrocera has discontinued the entire Contax line.
If anything in my arsenal of photo gear goes, it'll be the Mamiya RZ system first, then the C645 system ... the Hasselblad is here to stay, and I have recently upgraded all the lenses to CFi or CFE status and added any lens I had missing ... from 35 to 350 as well as 2 E type extenders.
When I order my next digital back it will be for the Hasselblad 500 series system ... which includes a 500SWC as well as the 503CW.