If you go 9mm go with Spikes. They are the easiest to set up and you have more options for mags. They use the standard mag catch. You also don't have to buy a different hammer.
Thanks for the info I didn't know that about the hammer. You still have to change the buffer right?
I think so but don't quote me on it. I do believe they offer one that can be used on both .223 and 9mm. It is a little bit heavier than a standard but will work with .223.
You can use CProducts, Colt, or Uzi mags. The Uzi mags will require a small cut with a Dremel to use the standard mag catch.
Do a little research on AR15.com and you will find a lot more info on them. I don't have one, but know a guy that does. He also said that Spikes are easier to set up. He researched all of the makers before he bought anything, and ended up with a Spikes. Hope this helps.