Hmmm..
You see.. the higher load rating on the same tyre(same brand + Size + Model) means one tyre can hold more PSI pressure than the other - essentially.
This would translate (on the same tyre) to the higher load rating having a stiffer build (increased use of rubber or whatever) and thus the tyre will be slightly more rigid.. and have slightly stiffer/stronger sidewalls.
Now when you take 2 of the same tyre.. and compare.. you dont need the higher load rating - unless you drive on roads which are not ideal (South Africa) and thus the stiffer sidewall may protect against tyre damage..
My view is (on the same tyre) get the one with the higher load rating, you might not know it, but its probably saving you more money than you think..