your right, he's scratching more because the fleas actually become more
active before they die.  it works on their nervous systems.  They basically
do the dance of death.  Frontline works in 12 hours.  also the frontline
treats for 3 months for fleas. (1 month for ticks)  I tend to go with the
product that lasts the longest, so I like frontline.  Advantage works just
as well, has less liquid aka less chemicals on your cat, but it does only
last 1 month.  It really is kinda your choice.