80 gb vs. 500 gb @ twin specs...What performes better?

The number pf plates makes no difference as each plate has it's own read/write head. The main thing effecting speed is the spin rate, so an 80 GB 10,000 rpm drive will be faster then a 500 GB 7200 rpm.
You won't notice any difference between smaller and larger drives in real use.
The larger drive has an advantage if you have an 80 GB drive that has 70 GB used as opposed to a 500 GB drive that has 70 GB used.
If you now try to copy 5 GB of files to the smaller drive it has to find space to put them, where as the larger one has plenty of space and can just put them in one spot instead of breaking them up to fit where ever it can.

Generally speaking, the 500 would be faster in "typical usage" scenarios as Jonmor is right, it won't fragment as easily.

Also, take age into account. A new 80GB 7200rpm drive will perform slightly better than a few year old equivalent 500GB. Newer manufacturing techniques, better components and such as well.