I have a 496 ho, i am thinking of either going with the cmi exhaust on the ho or trading up to a 500 efi. I am looking for suggestions.

I have no expierence with the 496. I think they are a great package. When you start upgrading there are a lot of things to consider. If you haven't already I would check with these guys on 496 upgrades.
http://www.raylarengine.com/

As for upgrading to 500efi I would go with the 525 if I had a choice.

All this really depends on what you are looking for or what your goal is in upgrading and you are willing to spend.

The closed cooling is a big plus for the 496. Like PS said, a 525 would be a better choice.