Wheeled robots come in a wide assortment of chassis. So all are feasible.
Consider the placement of the rest of the robot, the "brains" and the batteries. How do you wish them to fit together?
If you are running this outside, then you may have other concerns. You gain a little ground clearance with the motors on top but then all the wiring and the motors themselves are exposed and not protected by the plate.
Sometimes you just need to move forward and see what matters and what doesn't. For the most mundane applications it will matter little. Start pushing the limits of speed and terrain and these things matter.
For where you are now, I would say worry not about this (aside from mklrobo's comments on motor mounting, a +1 to him), but about other things, like what does the drive train geometry look like. Do you want a wide chassis or a narrow one. That is far more important in how a wheeled robot turns and handles.
Now, I have little interest in wheeled robots, but for the legged robot I'm working on, the placement of motors/servos for the center of gravity does matter. Not so much for you.
