I use AHS and Northern.
I like Northern's kits because everything is in a per kit box.
AHS kits, if you order 3, come in a big box with everything labeled, but jumbled together.
But that being said, AHS has many more kits, and their clones of commercial beers are awesome! So I have ordered more kits from AHS.
I have their juniper rye kit right now. Off the wall stuff like that is just not found at Northern.
You can spend HOURS here looking through commercial clone kits.
http://www.austinhomebrew.com/index.php?cPath=178_452_43
I have done a few and they all have been as good, if not better than the commercial beer.
I like how Northern has the PDF for their kits online so you can actually look at the recipe and decide if you want it, or if you want to order parts to tweak it.
Austin, you don't know until you get it, and you better not lose the directions.