The bottom right drawing is a bad idea, we tried somthing similiar, 6wd instead of 4, but you will never get the 2nd direction wheels to get traction, unless you use a suspension system. Due to the rollers on the wheels the frame never stays level long enough to gain traction with the 2nd direction wheels.
Go with the plastics ones, we tried the aluminium ones and once they are bent they are hard to get back straight, and make driveing in a straight line a nightmare, even with a gyro.
