I use Advantage, and I'll tell you why. I personally have one dog and one cat, but I live with a total of two dogs and four cats. If I treat just my two, there would still be fleas. I can't afford $60ish a month for all six animals, and if I don't buy it and treat them, it won't get done. Sooo I buy the biggest tubes of Advantage and split it up. Advantage is exactly the same for dogs and cats (Frontline isn't), just the dosage if different. I get all six animals treated from a little over two of the big tubes, i.e. it costs me just a little bit more for all six then it would cost for just my two if I bought the pre-dosed stuff.
I buy it from dog.com. Free shipping if you spend $59 (grrrr, it used to be $49), and one of the cheapest prices anyway. You need a couple syringes to dose it correctly, and possibly a needle to get it out of the tube (but obviously remove the needle before putting it on the animal).
I'll find the dosing chart if you're interested.