personally i like down B more.
but i think both are great in their own situations.
i definatly use down b more often for frontal attacks, mainly because:
-its way faster as it jets off initially, while over b jets after it lands
-its starts on the ground
-the great speed at start makes it hard to counter, because the counter is all about timing, and really only a handfull of moves can stop it anyway.
either spin dash can do a total of 3 hits from the ground (one is form the spin while its on the ground, then you jump and hit while spinning in the air, and then you cancel the spin into any ariel attack: forward A seems to be the best, but it just depends really), but if u hit with the over b while its in the air, you can only land 2 attacks in quick succession.
side b is definatly better for returning to the stage though... it still hops even in the air..
i eman really... you should use both anyway, i think the moves are different enough atleast in timing to catch people off gaurd with them.