My vet said he dosnt think there is any difference between the 2.
Frontline vs. Frontline ComboFrontline is the market leader in flea control and Merial have recently launched Frontline Combo. This combines the original flea killing power of Fipronil with an environmental treatment called S-methoprene which inhibits the development of flea eggs, flea larvae and pupae. Frontline and Frontline Combo are unique in that the flea does to need to bite your cat or dog to be killed; it actually kills on contact.
Over the last 15 years, fleas have evolved to be resistant to certain drugs. With this in mind, it is not only important to kill the fleas on your pet but to also break the lifecycle and prevent more resistance to drugs in the future. You will be doing this by using Frontline Combo.