That would be a really tough decision for me as well. I have a 5D mkI and love it. It would depend on how beat up the 5D you are looking at is. Like you, I would not use many of the bells and wistles on the 50D either. If the 5D in question looks like they took care of it I would probably go for the trade but if you could wait and save for a 5D mkII and keep the 50D for a back up do that. If you are going to do weddings you will need a backup body.