It won't matter on the interstate. They both have good angles so they will take bumps well. The difference is that the FT is a 4 link versus a radius arm, so it will have better caster through suspension travel, but you won't get enough suspension travel on the interstate to notice.
The FT will do rocks just fine, except the worse ground clearance. Capt. Nemo on the board here recently sold his FT kit in order to put on the TnT, mainly because of the better clearance since he wheels trails like the Rubicon.