A mod 40D will not only give you the benefit of LiveView but also better image data due to the 14 bit/channel as opposed to 12 bit/channel on the 20D.
IMO modding the 40D will give you the best modded DSLR body for astro imaging there is atm.
The 20D is still a fantastic body for astro though, it's up to you to decide if the extra benefits of the 40D are worth it.
I've stuck with my 20Da because it has LiveView and am very happy with it.
But if I was to choose today, the 40D would be at the top of my list.
