>>But the M1 Abrams and the Leopard 2 are over forty years old.

They've been constantly changing, being updated with items like more effective armor (most obvious difference between early Leopard 2s and current ones is the shape of the front of the turret), targeting systems (ability to fire a round while moving and actually hit targets, barrel stabilization, better accuracy against moving targets), communications systems, and (at least for the Abrams) better MPG; it's gone from about 0.6MPG to over 1.5). I'm sure I've missed some - and I'm no expert.

Both tanks are outgrowths of the joint US/West German development of the MBT-70. Both use the same Rheinmetall gun (licensed production in the US) and have interchangeable ammo. The Leopard 2 is more appropriate simply because there are already maintenance and repair depots in Europe.

TL;DR: These are pretty modern...