Borrowed this from an Amazon review... He/she sounds knowledgeable, and is correct about the ply ratings, so I'm going to parrot it here:
"The 115T signifies that the tire has a 4ply standard load sidewall. The 121S signifies a 10 ply load range E sidewall. The standard load tire is best for a half ton truck, whereas the load range E sidewall is best for a 3/4 and 1 ton truck."
So in short, yes you will be fine and should choose the 115T.
** Please note you are obligated to post pictures of the new tires after you get them installed!