Oh I love these debates!!
I believe the Leopard would probably be considered a better all around tank than the Abrams. It is doubtless more maneuverable, carries the same gun as the Abrams, and is more suited in some ways to maneuver warfare, being 10 tons lighter and able to cross softer terrain. It also requires less fuel and has a significantly improved range over the Abrams, almost twice as far in fact. The Leopard is a diesel, the Abrams a Jet Turbine, so the Abrams is faster, but just a bit. I believe the Leopard is also cheaper.
That being said, it has less armor protection than the Abrams. The trade off being that agility will make up for risk of potential hits on the turret or hull. Nothing wrong with that trade off. It's a bit ironic though that this is the same trade off the U.S. made with the M4 Sherman over the German heavies of WWII.
Something odd to remember though, the Abrams is primarily a DEFENSIVE weapon. The Leopard is designed for a more OFFENSIVE role, or at least rapid EXPLOITATION. One thing I dislike about these various comparisons is that they never peel back the onion to discover the actual mission the weapon is designed to accomplish.
In it's doctrinal role, the Abrams engages targets from a defensive position, and is designed to absorb damage while at the same time inflicting it. It's speed was needed to move it from position to position rapidly, where it would once again engage targets as part of a coordinated defense. The fact that the U.S. has used it in primarily the offense is incidental, but you see the 2nd and 3rd order effects in the massive amounts of logistics support required to support the tank in the offense. That creates a vulnerability, which while shared with all armored forces, is particularly notable when using the Abrams.
However, Brad Kaste makes a perfectly legitimate point...the Abrams has a stellar tank on tank maneuver warfare record. There just aren't a lot of things out there that can kill it. The Leopard has been used in combat, but cannot even come close to making the same claims. I've not seen a lot of kill rings on Leopard 2 tanks. There's a motor pool just up the road from me, where I can show you dozens of Abrams with kill rings -- and these are tank on tank kills.