I wouldn't use either one for a passenger car except in a dire emergency, but I do use Slime on my ATV. I use it as more of a preventative than anything else. I put a bottle in each tire every 3-4 years. In my experience it works well. It seems to permanently plug slow leaks, and turn regular leaks into slow leaks. It is especially good for troublesome leaks around the rim. I have never had much luck with Fixaflat. It's too watery. As others have said, you are better off getting a plug kit along with a small portable compressor.