Advantage! If you're infested ask your vet for capstar, which is a pill that kills live fleas within a 1/2 hour. Then use the topical every 2 weeks to kill the life cycle. Most of the OTC stuff like Harry or top spot and zodiac can make pets really sick. I've seen it first hand, spend a little extra to avoid a catastrophe
The tablets are the most effective at killing adult fleas so are a good choice if your cat is already infested. The spot ons work on the flea life cycle and may be a better option if you are looking for a preventative.