X2's are more bass heavy, probaby best suited for playing really any music since they are a bit warmer, but if you are like me and already own a pair of warm sounding headphones and want a colder pair for anything else(or listening too) go for it.
If the K712 are the same as the K7XX, then I'd say the Fidelio. I've heard them both but you'll pay a $100 premium for the Fidelio X2's. I already have the K533 which still has the AKG sound.
I have both, and would get the X2. I got it for not much more than the K7XX so it was a great deal. Both are great but Build, comfort and sound I prefer the X2.