There’s not much in it, but a square may be slightly better

Modern PDA and square reserves are both quick in deployment and stable as they descend. But the flatter shape of the typical square reserve may make it slightly quicker to deploy, and stabilise faster.

Another aspect of square reserves is that they do tend to track in one direction or another. This has two consequences. The first is that the horizontal tracking creates lift, resulting in a slightly lower descent rate for the same size, allowing squares to be slightly smaller and lighter to achieve the same descent rate. The second is that the tracking could mean that there is a greater (or lower) horizontal speed on landing, since the direction of track is random, so it could add to or reduce effect of the prevailing wind.