Since MultiFinder17 answered most of the questions, I'll just do the pros/cons over the 6400.

Pros:

Higher processor clocks

50MHz system bus (compared to 40MHz on the 6400)

Improved performance in disk and other I/O operations (newer system controllers)

Integrated ATI RAGE (I think it's a IIc) video controller with 2MB of SGRAM (compared to the old Apple custom chip in the 6400)

Cons:

None, unless you'll really miss that extra 8MB of RAM that isn't soldered to the motherboard anymore.

Plus, the Gazelle models can't run OS X. But then, no 603-based Mac other than the 3400/2400 can run OS X.