The first is Load Range C and has black letters on both sides. The second is load range SL and has white letters.
Here's Dunlop's page on the RT:
http://www.dunloptire.com/products/tire.html?tire=radial_rover_rt It has a typo in the heading for the 4th column, it should be max load for passenger car use.
Load ratings explained .
What are you doing looking at the dinky little 235s anyway?
Get the 31s!
Brent