depends on how much non-astro work you are going to be doing with it. If it is mostly going to be for astro - then spend your money on a 2nd hand QHY8 - or even a new one if you get it from Peter Tan (
www.tan14.com). These are cooled, and have several orders of magnitude less noise than a DSLR.
Buuut...if like me you are doing a reasonably even mix of terrestrial / astro, then there really is no choice, until you can afford both...
When I bought my 550D, the salesperson told me that the 550 had less high iso / long exposure noise than the 500, and that the 550 was basically 7D innards in a cheaper (non-titanium/non-weather-sealed) case. How true this is I haven't really tried to find out....
Adam