When making butt joints for corners... structurally is there any difference between longer tops and longer sides [using biscuits and glue]
Yes, with longer sides it would be stronger. The reasons being:
- for the glue line to be broken it has to shear through at 0°, versus being pulled apart at 90° which is much easier (although it must be stated, still very difficult if made properly);
- and with biscuits they'd have to be snapped across their centre for the joint to break, rather than simply pulled free from their slots which is obviously much easier in principle (again, still very difficult).
For very large constructions that need to hold up a lot of inherent weight (the weight of the piece itself), even before the added load of items stored inside, longer sides would be far preferable. But for smaller pieces I suspect it doesn't matter in practice.
One additional advantage to longer sides is pure aesthetic: in this case (wall-mounted item) there's less end grain visible in the finished piece, which is considered very desirable by many woodworkers although some are less fussy about this.
A lesser question: In addition to structural properties, is there any traditional/historical stylistic tendency for longer tops vs. sides?
It wouldn't have been common for boxes of this basic construction to have been built historically as they didn't often rely on 90° butt joints, but in more recent tradition where dowels have been widely used I'm fairly sure longer sides would have been the preferred orientation for exactly the same reasons as stated above for biscuits, where again the dowels can be pulled free more easily than snapped cleanly in two.
It shouldn't go unsaid though that many traditional woodworking techniques go toward making the strongest piece possible for any given joint type, and it's widely acknowledged that this means much furniture has over-engineered joints, i.e. much stronger than truly needed.