Well, actually that's really debatable. Since both the Leo2 and the Abrams use the Rheinmetall 120mm cannon and both use Armor Piercing Fin Stabilized Discarding Sabot-Tracer rounds. This stuff is incredible. No explosives, using only the kinetic energy to pierce through any armor. The kinetic energy is about the same as a 100t locomotive at 50km/h has, concentrated on a small spot!
Look at this:

However, the Leo can aim better. It can compensate for it's own and the target's movement, and can even hit a chopper moving at 250km/h 3km away. Sure, Abrams can compensate too, but is no match for Leos "Feuerleitanlage"
So it boils down to the armor, detectability and mobility. The Abrams uses a gas turbine, the Leo2 a Diesel engine. While the turbine may have a bigger power output, it's not as reliable and much more power hungry. Leos Diesel engine can be fed with diesel, kerosine, petroleum, vegetable oil and about everything that burns, including whiskey.
Abram's Camouflage may look incredible, but it's not as good as Leo's adaptive one. Also, the Leo is able to create a screen of smoke, which allows him to escape visual and IR detection
I can't say much about the armor. It's a secret on both side, but I do know that the Abrams uses depleted uranium, which is about the most dense material you can use for this, while the Leo has CHOBHAM-multi layer armor. Leo's one is much thicker tho, especially at the front.
So yes, it's debatable :p
(plus, you can cool 2 12-packs in the biological/chemical warfare filter compartment of the Leo)