The larger harddrive space is always nice so if you can afford it I'd get the bigger one.
As far as the case goes, a non-elastic case won't fit both, the 160 gig one is a little bit thicker than the 80 gig one and a very snug fitting case for the 80 is unlikely to fit the 160.
Congrats on the Xmas gift and even bigger congrats on saving/earning the money to pay part of the expenses yourself.
Dr. Mom (Who's always happy when her 16 year old saves the money she earns working to get something she wants rather than just asking for it. I give mega 'brownie points' to any teen who has the moxie to work and save for what they want rather than just asking for it)