I found these bulbs for my 98 cavalier and I noticed the different power ratings...

This one is rated at 55w
http://cgi.ebay.com/ebaymotors/ws/eBayISAPI.dll?ViewItem&item=7927751754&rd=1&sspagename=STRK%3AMEWA%3AIT

and this one at 80w
http://cgi.ebay.com/ebaymotors/ws/eBayISAPI.dll?ViewItem&item=7927686723&rd=1&sspagename=STRK%3AMEWA%3AIT

What is the differences? Which one should I buy?

here's an easy comparrison.... look at a ligh bulb in your house.... Find ones with differnt wattage... turn them on... whats looks different about them?... ONE IS BRIGHTER THAN THE OTHER ONE!! Come on now, this isn't rock science here... my 9005 Sylvania Silver stars run 65 w and the 9006 runs 55w... personally i'd invest in headlights that are a little more reliable than e-bay lights... If they short out while your driving at night.... your screwed... pay a few more bucks and get the silver stars or PIAA makes some good bulbs as well.