Despite the advantages and disadvantages of any drive, I'd suggest waiting until you know your challenge before selecting any aspect of your robot design, with the possible exception of electronics boards (although for reasons best discussed elsewhere, I'd warn you against even that this upcoming year).
Also, keep in mind that Mecanum drives are not the only drive solution that allows for strafing. Both swerve drives and holonomic ("omni") drives can achieve it, and have different advantages and disadvantages. I'd suggest searching around the forums for different threads and whitepapers dealing with each and make your decision based on the information gathered during that search and what the game demands.
Here are some useful starting points:
http://www.chiefdelphi.com/media/papers/1836
http://www.chiefdelphi.com/forums/sh...ad.php?t=57282
http://www.chiefdelphi.com/forums/sh...ad.php?t=51582
http://www.chiefdelphi.com/forums/sh...ad.php?t=48817
http://www.chiefdelphi.com/forums/sh...ad.php?t=48637
http://www.chiefdelphi.com/forums/sh...ad.php?t=41868
http://www.chiefdelphi.com/forums/sh...ad.php?t=49705
http://www.chiefdelphi.com/forums/sh...ad.php?t=38370
http://www.chiefdelphi.com/forums/sh...ad.php?t=36205