I made the switch from a 350D to a 400D and the biggest factor for me was the rear screen size. Now I know none of us chimp our shots, but if we did, that screen would make a difference 
Seriously, you are never going to see the true quality of your shot from the rear screen but with the 400D you can see it better. I shoot RAW, use extensive bracketing etc but I still don't feel confident to move on until I've seen the composition on the camera screen. So for me, that was a big plus - for you a 350D may tick all your boxes.
Either way, both are great cameras and you should be pleased to own either. Have great fun and remember to shoot some ducks 