Jazz1,
I asked the exact same question on the other X2 thread. Mad Lust Envy responded thusly:
"The AKGs are warmer and smoother sounding. Both have good soundstages. The imaging on the X2 is sharper, as the AkG is more fleshed out but more diffused. I honestly dont see a reason to have both. Both are great cans, but both work well as all rounders.
if you love the X2, dont worry about the AKGs, trust me. The X2 is a headphone that doesnt really need an alternative, unless you wanna lean heavily on warmth, or on detail orientation. Having two all rounders isnt practical.
If anything, I'd compliment it with a good closed headphone."
My takeaway is you can't go wrong with either one.