42" not only provides you with extra storage space but will be more in keeping the proportions of the 10 ft ceiling.

I agree with Dytecture. Unless you raise them higher for under cabinet use, go with the larger one. Check out IKEA for what I mean about under cabinet use. Standard cabinets do not allow for the use of rail systems. Plan what you want to do below to figure out height to install at.

Consider specking toy box hinges and making the top cabinet have awning type doors. (stays open while you load or unload the space) I will also do glass inserts and Led up lights for mood lighting and lighted display pieces.

You can save $ by going with 56" and just have the door be two panel with the top panel being glass.