The 20D will always have better image quality in terms of noise at higher ISO's so as the astro cam, it would excel.. on the other hand, it would excel for terrestrial imaging also... I'd probably go the other way around and mod the 20D, but thats because im an astro nut!
Congrats on the new toy mate! you will be very happy with it, regardless of what task you dedicate it to.
Alex.