Comparing apples to apples (Omni-Wheels in an X Pattern and Mecanum in Standard Pattern so that both are fully holonomic), ignoring the quality of vex parts and the user’s design and build, there ARE differences between them.
Specifically, with an “X-Drive”, your cardinal direction speed is increased and torque is decreased, and your diagonals are at regular drive speed and torque, then with a Mecanum Drive, it is the opposite, with your cardinal directions being at regular drive speed and torque, but your diagonals having increased speed and decreased torque.