Here we go...
Clayton's is superior, but bottom line is that you can't really go wrong with either.
The main reason Clayton is superior is the incorporation of a subframe, which when dealing with a unibody vehicle, its value cannot be understated. The adjustability of Clayton's is also superior to IRO considering IRO continues to use that hokey caster adjustment bracket. I also prefer the rear 4 link design of Clayton's versus IRO's retention of the a-arm design.
Square vs. round is a never-ending debate, and if you're really concerned about that issue, head over to Pirate for some good reading.