I recently sold my aluminum Macbook Pro 2.4GHz 13" and bought aluminum 2.8GHz 17".
I use/d both 13" and 17" with 8Gb of memory and SSD installed.
Performance difference IS minimal.
13" is lighter and smaller.
The 13" has one fan and can get very hot and ever freeze in warm climate, while 17" has two fans and never stops.
Both have very long batteries, the 17" is a bit longer. I think MFDBs draw too much power from FireWire, so it's better to connect Macbook to a power outlet for a long tethered work.