1:7, 1:8 and 1:9 will work with most 5.56/.223 ammo the average person will use. I would read carefully what kraigwy wrote before choosing a twist rate.
For most of us who won't be going much under 55gr ammo, the 1:7 twist makes a lot of sense. I have three ARs with 1:7 and one in 1:9 and really can't see much difference with 52gr ammo (I haven't tried anything lighter). There definitely can be a disadvantage with heavier ammo and 1:9.