The main difference between Frontline Spot-On and Frontline Plus is that Frontline Plus contains an additional active ingredient, (S)-methoprene, which prevents flea eggs and larvae from developing. This means that Frontline Plus can provide more comprehensive protection against fleas, as it can help to break the flea lifecycle.
However, Frontline Spot-On is generally less expensive than Frontline Plus. So, if you are looking for a more affordable option, Frontline Spot-On may be a good choice for you.