[Lil' Lavery]

it all depends on your robot and what roles you want to play on the field. A holonomic or mechanum drive system is going to have problems with the ramp unless you create some sort of suspension, although they will be the most agile and mobile in the open field.
A tank drive will have tons of traction, and be awesome for the ramp/platform, but it may not possess the maneuverability to an omni-drive in certain situations in the open field, or chase bouncing balls.
Swerve drive may be a nice solution to these problems, but those can be fairly hard to build.

There are many many other factors that tie into this, what kind of ball shooting are you employing, will you shoot on the move, will you shoot at all, etc etc etc