They are pretty much the same tank IMO, as pointed out by others they both evolved from the MBT-70 program or whatever it was called what was a joint German-American Army project. I will also stick with the battle hardened theory because that was also the reason that the Abrams and Challenger 2 received first round bye and until the Leopard 2 has some combat statistics to back up its stats on paper I will go with the Abrams. Remember, teh T-72 on paper looked like an awesome tank for the Russians but it was so ineffective in Afghanistan that the Soviets used T-64's instead.
Please note that 98% of what I say is my opinion and/or my "version" of the facts. Most of what I say is rumor with little to no evidence to back it up, just something I picked up somewhere.