I just bought an AT&T phone and bought the Net 10 sim card. I live in a really small, rural town and I get 3g most of the time.
As far as which thing you should select while ordering the sim card, select the AT&T one unless your phone is actually an unlock one.
I will tell you that you will still have all the AT&T stuff on your phone and I have yet to find away to get it off mine. It takes up a lot of space. Also I have problems with it sometimes switching back to the AT&T APN. It's kind of a pain, but I can't figure out how to delete the AT&T one. I even call Net 10 and they told me to call the company that made my phone. As far as I've read, there is no way to remove it short of rooting your phone. Which I can't do.