See the current thread "how to move from deeps to mediums". When you first start out it would make more sense to run all mediums. If you change your mind you can just convert the medium brood comb to honey comb supers and add deep brood chambers.

On the other hand you cannot as easily switch to mediums from deeps. Not nearly as hard as putting toothpaste back in the tube, but still a lot more complicated than just adding a second size.

I run all mediums. Three mediums for brood and the bees, everything above the third box is honey. I do not use excluders