I visited both web sites, and saw right away what the difference is:
K9 Advantix claims to stop fleas from biting in 5 minutes and kill them within the hour; it claims to repel ticks so that ticks do NOT attach. It also kills mosquitos on contact.
Frontline claims to kill 100% of fleas on your pet within 18 hours, and 100% of ticks within 48 hours.
I use Frontline and recently pulled a tick off of Google; I was freaked out. It hadn't had the opportunity to feed yet, and my vet told me that judging from what I told him, there was no possibility that Google was infected with Lyme's disease. A tick has to be attached for 12 hours before Lyme's is transmitted.
So, K9 Advantix prevents a tick from attaching in the first place. Frontline only kills the tick after it starts to feed and has transmitted Lyme's disease.
Google's been using Frontline, but after reading this I plan on switching.
web site for k9 Advantix:
http://www.k9advantix.com/
web site for Frontline:
http://frontline.us.merial.com/home/