jadatis and pnichols:
While it may seem reasonable to assume that a higher Load Range tire for load carrying capacity because it is stiffer (or alternatively, for durability, ruggedness or whatever word you want to use), the problem is that this difference in stiffness is small compared to the difference in stiffness inflation pressure makes - and for practical purposes isn't a difference at all.
Stated another way, there is a reason why the load charts show the same load carrying capacity at a given pressure regardless of the Load Range (except,obviously for the maximums!). That reason is that, for practical purposes, you can't detect a difference.