"AR" (per actual designation) is Armalite Rifle and the numbers refer to model numbers, most commonly AR-10 and AR-15, Eugene Stoner's rifle designs (in 7.62x51 and 5.56x45 respectively), the rights to which were eventually sold to Colt (the AR-15 platform led to the M16 and M4 series carbines).
"SR" is Knight Armaments Company's (KAC) designation for Stoner-based rifles/carbines, the SR-15 (and SR-16) series being 5.56x45 and SR-25 being 7.62x51. There's also the short-lived SR-47, chambered to 7.62x39, which accepts AK mags.
There's a really long list of AR variants, specially with Colt, following the entire evolution of the gun from the original Stoner design to the newest M16A4s and beyond. Here's a partial list if you're really curious (I've seen more exhaustive ones):
http://en.wikipedia.org/wiki/AR-15_variants
That doesn't even account for all the AR based designs from smaller shops or variants with addon rails systems, etc.
Basically it all comes down to looks. Unless you're looking to get something that fits a specific impression (Navy SEALs, USMC, CF, etc) just get whatever you think is most purdy.