To be fair, I think they're both equal in term of perfurmance (Omnia has faster processor while Xperia has larger ram) and the OpenGL in Xperia is already taken by the WVGA screen (while in Omnia it WQVGA with no OpenGL) but it still support DivX.
Omnia outstands the Xperia in the camera, including it's flash (although it's Sony's speciality) and Xperia outstands Omina in the display (monitor).
Xperia UI (Panels) is way better than Omnia's (TouchWiz); they're both handy and finger-friendly but TouchWiz is uncustomizable.
Also, Xperia has a full keyboard and more buttons on the outside while Omnia has G-Sensor (Accelerometer) and less in weight, size and price.
Finally, Xperia is better when it comes to SSD Memory Card (don't have to remove the battery) and Stylus (stuck inside instead of hanging on the side). But with the 8 GB inside Omnia, no one needs SSD Memory Card and the interfaces in both are already finger-freindly so through away your styluses.