I shudder at the thought of Mecanum with only 2 CIMs. You might as well disable trying to rotate while moving in any transverse direction. You also might as well disable the diagonal movements. The wheels simple eat up too much torque due to the 45-degree roller offset.
Simply put, either drive train gives a good driver very similar capabilities. Crab gives the driver an edge in finely tuned movements, yet Mecanum may give the driver more practice since it's much simpler to build.
With Mecanum, square up on another robot before pushing it. Programming is relatively simple. The Toughbox Nano's make a Mecanum drive train easily constructable since 4 nano's with cantilevered Mecanum wheels should be able to support the entire weight of the robot without issue.
Seriously, If AM redesigned the toughbox with a steel box extrusion like what the nano has, they would probably be strong enough for military-grade and/or volatile environment (think chernoble cleanup) robots.
With Crab, your driver needs to dictate the driving style, which then dictates how the 4 wheels are steered (2-2 sides, 2-2 front/back, or all together) as well as how the robot is programmed. This driver would definitely be more focused on the robot during a match and less focused on what's going on around him/her.
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I'll be honest, I have nothing quantitative to share with you, but...A really good 6 wheel skid is hard to be beat. It's lighter than a swerve and if you look at the teams that do it really well, their performance really isn't that much worse if worse at all from a good swerve. Keep the driver in mind too, different drivers prefer different systems. Skid steer requires basic code, successful swerves require much more complex code, especially if you want it to be intuitive.
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It's all about
how you use the drive train. A 6-wheel drive train has shortcomings when attempting maneuvers, especially in autonomous. For great examples of what I'm talking about, look at Wildstang's 2007 and 2009 robots. Both autonomous routines used maneuvers with swerve that would have probably been much less accurate (statistically) than had the maneuvers been performed with skid -- especially in 2009.