As people are pointing out, it is a crap shoot if YOUR rifle will need small base dies or not. SOME auto-loading, pump, lever, etc. rifles seem to need them, some don't. The only way to tell is buy a set of dies and try them. Then if you need the SB dies, you buy the SB dies, if not, go on your way. Of course it you DO need the SB dies you are out twice, so it might be good to buy them to begin with and not worry if you really needed them or not.
The RCBS AR dies resized the case farther down the body (as do all SB dies) and set the shoulder back slightly so your cases will chamber in a slightly dirty chamber (the AR dumps a lot of gas back there) and ensures the case won't be too long and jam.
Bullet length and ogive are also important. I was down at the gun shop and the owner asked me what was wrong with his .223 reloads as they would not chamber properly. I said if they were sized right (he claimed they were), if the neck or shoulder wasn't buckled (they weren't), then it was the bullet. He claimed he measured them and they were the proper OAL. I said the ogive was hitting then, shorten them up! He gave me a look, went to the bench and shortened them up a few thousands and guess what? They worked!
Hope this helps.