The primary difference between Advantage and Frontline is that Frontline kills both fleas and ticks, and Advantage just kills fleas.
I use Advantage for the cats because for fleas it just seems to be more effective, but I use Frontline for the dogs because they get ticks, and they swim (Frontline is also more water-resistant, but who has cats that swim?). My cats don't pick up ticks, but if you ever see any, try using Frontline.