Okay boys and girls, botton line it for me- Should I buy a flat-top upper receiver, or one with the sights and the carrying handle? My main use would be varmint hunting/target shooting. Are the receivers with the handle more difficult to shoot with the addition of a scope, and if so do those slip on cheek pieces help? I figure the price is the same either way so that isn't a factor.

Since you mention your primary use as varmint hunting I would opt for the A3 version as there are many more scope mounting options and more types of mounts available than for the A2 w/ carry handle.

I have owned an A2 w/ scope and cheek piece but it never felt right to me.  Flat tops are much more suited for optics and I have better accuracy as the scope is closer to line of sight on the weapon.

You could just solve your dilemma by getting an upper with the removable carry handle.  This way you could experiement and arrive at what is best for you.

Flat top!