Assuming that this is your 1st AR, I'd suggest the A2 for:
Simplicity
Durability
Repeatability
Also:
The ability to mount optics on a forward mount, and for some setups, this is a better option than the flat-top mount.
The possibility of the eventual urge to match yourself against other shooters in Service Rifle competiton. You'll be unable to compete with an A3. On the other hand, you may decide to buy a NM upper for this reason?
The tendency for people to neglect gaining a proficiency with the iron sights if they've got the option to use a scope.
I dunno, up to you, but I tend to think that unless someone is building a devoted varmint rifle, or unless they are one of the groupies who wish to look like active duty servicemen, that they ought to start with the A2 upper.