Why is square better?
U and V shapes for the bottom of rear sights have several things going against them.
1. Light transmission is the biggie. Easier to align irons when light transmission on the sides is uniform, top to bottom. It also helps if there is a decent amount of distance between front sight post and rear sight "arm." So that even when the rear is out of focus, a change in light intensity from one side or the other indicates misalignment.
2. When being used at ranges greater than is usual (beyond 25 yards) this is more critical. If you've marked your front sight with appropriate range bars, being able to see the whole range bar in the bottom of the rear sight aids in precision. And having the entirety of the front sight post width visible at the bottom of the rear sight keeps the shooter from having to work harder to align the V or U shaped (rather than bar shaped) mark along the width of the front sight. Especially once you've transitioned to your final focus on the front sight post and the rear is now blurry.
I also am not fond of front sights that are not rectilinear. I don't mind if they incorporate a dot or circle within the rectilinear sight body, but I don't want an angled or curved side or top. That last part goes for rears, too. Dots, outlines, different colors, whatever. They are fine as long as the meat and potatoes of good rectilinear sights are present.