I agree that I'd like to get the largest one that will fit without clearance issues. The tread is so similar between the two that it really comes down to size for me and which one clears the AWS system and deck the best.
I'd like to go with the 26x12x12, I just want to make sure that they'll clear properly with the JD spacers. There was another member that made 3/8" thick spacers for his AWS 4X5 and it seems to work with the 26x12x12 HD Field Trax. I was going to us the JD spacers which are 1" thick, and have questions on the clearance of the 26x12x12's with the 54" rear deck side rollers.