Here's a simple break down:
DT990 - Brighter treble, a little more mid-bass (compared to X1). Replaceable ear pads.
X1 - Pads are not replaceable.
Both will run from the STX with ease. The X1 will run from your laptop with ease. The DT990 will be variable based on which version you get (32ohm, 250ohm, 600ohm) so is more limited with the laptop depending which one you get.
Amazon is a great place to buy either. Great return policy. And you can get...