[wireties]

We have used both mecanum and swerve. The mecanum is easier to build provided you pay attention to a few design rules. All 4 wheels need to be in the same plane thus excellent tolerances on the frame design and/or mounting the wheels with shocks/springs of some sort is essential. Keep the mass above each wheel about the same thus symmetrical distribution of robot components (their mass) is critical (don't forget the battery). Finally we had much better performance with encoders on each wheel. If you design the robot keeping these principals in mind the software is pretty simple.

We built mecanum because the students loved to drive it. You will hear many biased opinions about mecanum. But I would have to agree that when playing against bots at the Einstein level the disadvantages are more serious. At the regional level, mecanum is fine and it is fun. A good driver (with some practice) can move around or spin around all but the top tier tank drive setups.

Swerve drive is awesome but quite complicated. It requires more parts, tighter tolerances, more motors etc. Many others in this thread make good points about the complexity. But it is awesome when you get it working! We stopped making swerves when our expert machining mentor became ill and couldn't help with the manufacture.

Either is a worthy off-season effort. Good luck!