i would always go for one sided shelves because two sided shelves just drive you nuts with things getting pushed through to the other side - unless u have it against a wall, but if u have it against a wall, it's still a one sided shelf
if you're having shelves built - think about what it would look like if you have two shelves (same size) back to back. if you had the shelf built so that both sides were open with a "wall" in between....u could use the same shelf for two different play areas by putting the end against a wall. one side could store dramatic play items and the other blocks.
if you're using a shelf for "cubbies" i'd have them mounted high up on the wall so the kids can't go and pull things down (without standing on something...which they'll do)!