Used 5DM1 vs 50D

I am considering either a 5DM1 or 50D for portraits and macro work. They are both selling used in the $450 to $550 range. I've been assuming that the full frame 5D would be the better choice due to the full frame sensor and that it probably has a better build. Is there any reason one would go with a 50D over the first generation 5D?  Thanks in advance for your help.