I remember reading somewhere about the mini's and reading CD's at angles other than the ground. Might be a consideration.
Also keep in mind it may be flat when you open it out but when it gets stuffed in and closes it is going to be at another extreme angle.
Aesthetically when open, I think the angle (resting on base NOT parallel to floor) looks best. Also if you will be inserting CD's and such, it seems more logical for a car environment like it should be.