I agree with Don; if you haven't already built a crab/swerve or have a really solid idea of how to do it, don't. Crab/swerve is probably the most mechanically complex drive system you could try, and the software is even worse. I also agree with Tyler, mecanum is probably the simplest way to a omni system, but it still will require significant amounts of research. The bottom line is, do you want to spend six weeks on a drive system that you've never tried before and risk completely failing, or do you want something tried and true that can be complete by week two (or even before, depending on shipping times)? If you;re adamant on using an omnidirectional system, the PowerPoint on them at the
FIRST Robotics Canada Resources Site would be a good start.