40D all the way. S/N Ratio will be better, the body is a lot sturdier, more custom adjustment options which can all be useful... and for the meager price difference, you may as well...
When you say "astro use mainly"... Do you mean, it may on extremely rare occasions be removed from the scope. If so, buy a cheap point and shoot for happy snaps and save more pennies towards a dedicated astrophotography CCD imager.
My 2c.