Seems a bit misleading to me. Why is the AE ring not a complete 'sine' wave and why would I pay any additional amount to get an additional few microns of flatness?

Once the diff is done up to the correct end load, either of these minor errors will be eradicated by the diff ring being pushed flat against the balls - if it weren't then the diff wouldn't work! I'm with Bertottius, get a pair of AE rings, fit the right way round and enjoy the glass-smooth action - providing you've built the rest of it right!