[mathking]

We have experience with both mecanum and omni wheels. We have not made an omni wheeled omni-drive competition robot but have built prototypes. So a few observations you can take for what their worth:
(1) Both types are harder to control with uneven loads. But remember, so is tank drive with traction wheels. It's a matter of degree.
(2) Mecanum tends to be harder to strafe in a straight line.
(3) Omni wheeled designs tend to be more difficult to control when you are turning the robot under load.
(4) Both 2 & 3 can be compensated for, at least to a degree, in code by using a gyroscope. (I have found it is easier to compensate for the strafing.)
(5) A good driver who gets to practice a lot will tend to be able to much better control their robot with any drive train.
(6) Because of 5, driver practice, in my experience, trumps drive train type in teleop.
(7) This year various slide drives (those with one or more centrally mounted wheels perpendicular to four omni wheels) are going to have control difficulty with the scoring platform unless you are very mechanically adept. This is not insurmountable but it is a consideration.
(8) You want to get your drive train done as quickly as you can, but not so quickly that it is not robust.
(9) When you have a working drive train, particularly if you are using code to help it drive straight, there will be tension between programmers wanting to program (and test) it and drivers wanting to practice. Make sure you come up with a schedule to assure adequate access.