Always buy the best you can afford! And Leupold glass is about the best there is IMHO.
I have three Leupolds, and I wouldn't trade them for any scope out there. I have never had any problem with my Leupolds tracking. They have always tracked as they should. Movements were precise, and stay...
I have three AR's. Two have the standard 20" barrels, and my Stag Arms Model 6 has a 24 inch, heavy stainless steel barrel. The Stag is by far my favorite. Having said that, it is going to come down to what you are comfortable with. If you are going to be shooting offhand, a heavy 24"...