For a small square, you really shouldn't be able to tell any difference in accuracy of a square. A spec of sawdust, the angle you hold your pencil/knife at, etc should make a much greater difference than the quality of the manufacturing. If you have a good standard, like a non adjustable machined square
from rockler or other stores, you can just check any old square to verify it. I've used starret, mitutoyo, brown & sharp at work, all seem about the same to me. I use a harbor freight at home, haven't really noticed a real difference in the tool. Better quality control and verified accuracy are probably the only difference. You can do the same by verifying it with an accurate standard. I do have a starrett protractor at work, and I am partial to it. It's extremely well marked/defined, holds position well, much better than any other protractor with sliding rule that I've seen.
I've always wanted a really nice 30-40" square for setting straight edges to cross cut a full sheet of plywood, but something that big that's accurate (think .002" per ft, not per in) is finicky and if you drop it, odds are it's junk. On a 48" cut, it would be like .008" off and that's really not noticable. .001"/in would be 3/64" off across a sheet of plywood and that's very noticable.
As far as parts, I havent' tried but I wouldn't be supprised if starret parts dont' fit all the other combination squares out there.
I would strongly recomend a good fixed square with a good accuracy, .001"/ft ideally. Use that to verify whatever square you have, check it occasionally, check it if you drop it, etc. Keep the accurate but affordable non adjustable square in a safe place.