The USHL is the only Tier I league in the U.S. The NAHL is a Tier II, Junior A league (there's also TII, Junior B & C leagues). There's some talent in the NAHL, but the USHL is much more stacked. USAHockey.com has some good info on all of the junior leagues:
http://www.usahockey.com/junior_council/main/junior__a_/