All I've really heard is that the Subaru OEM aero cross bar kit is quieter but not as strong/stiff. The biggest problem is that it's more trouble finding aftermarket mounts that fit onto it - you'll have to get adapters for some racks, and others just won't work.

If you're getting aftermarket crossbars, most people say you can't go wrong with Yakima or Thule, but they do cost more. I'd stick with the standard round/square just for compatibility and fewer hassles.