I think the Russians might have already taken out 6 Abrams, 5 for sure. The Ukies got the 80s-90s versions, which the Russians say are easy to destroy. From what I've read they consider the Leopard as the best NATO tank they've faced.
One great aspect of the Abrams and Challengers failing so bad is that it will spare Ukrainian land from being poisoned forever by their depleted uranium ammo.
The decision to send the Abrams to ukraine was political in nature - we had to step up ourselves, to push Germany to green-light the transfer of Leopard 2s. So we've sent only 31 tanks, of the old M1A1 variant, with an upgrade package, but still below, the already inferior, export models. Not that it would make a significant differance (well, better optics would come in handy for long range shots) because none of the currently used tanks in the world have been designed with the kind of threats we see in ukraine, in mind. A commercial FPV drone worth couple of hundred dollars, with an RPG warhead strapped to it, can disable a tank worth millions of dollars, regardless of its make and model - just hitting its sensors, treads or the engine cover. Still, the Abrams is even more vulnerable to those drone attacks than other tanks: the blowout panels (thin plates bolted above the ammo compartment) atop of the turret are a clearly visible and obvious target for drone operators:
Photo: Abrams No.2, that got hit in its treads.
You can see the blowout panels intact - pretty easy for a drone operator to aim for them.
Is the Leopard 2 better than the Abrams ? Ukraine received more modern Leopards than those M1A1s, so they have better capabilities, and they've been delivered in a greater number - around 100.
Personally, I think that T-72s are the best tanks that ukraine received - not because of their capabilities, but because of the sheer number of them, and the availability of spare parts and ammo.