I upgraded 350D to 30D (I use 99% M mode)
The image quality is very similar, not better or worse on either just a little different (hard for me to describe how) barely if at all noticeable.
The 30D is just nicer (little faster) to use, feels more substantial.
I have used spot metering quite a lot so far and can see benefit.
1/3 stop ISO can be useful too but no deal breaker and the joystick is a far better way of changing focal points.
If your shooting fast, ie candids/kids or sports, the 30D will give you more keepers. (assuming a decent lens)
The picture styles are handy now I am more competent/confident to shoot jpg (I use this for candids and family snaps sometimes, 0 pp needed.
Anyway, thats my experience from upgrading, good luck with whatever you choose to do.
There are 10 types of people that understand binary, those that can and those that can't.