Poor bun. I just had a bun with a bad flea infestation and I used advantage. Probably came in on me from the dogs. Like Ali said both will work fine. I placed on drop on behind her neck and within a couple of days saw a great reduction in the flea population. I did noticed that most moved to her rump then, so I put a drop on back around her hips.
I flea combed her everyday. When I did, I had a cool whip tub with water and dish soap in it. When I combed her and found fleas I would push them off the comb into the water. I didn't dip the comb in the water, the soapbreaks the surface tension of the water and the fleas sink to the bottom.
Between the advantage and the flea combing it cleared up within a couple of weeks.
Good luck.