I-75 may be shorter and flatter, however you will probably encounter more traffic. The only major grade you will encounter on I-24 is Mt. Eagle, about half way between Chattanooga and Nashville. It's 6 percent grade for about five miles up and five miles down. We tow over it all the time with no issues. I-65 north of Nashville through Kentucky is hilly,but mostly flat.
M & M :C On the road again!
2007 GMC 3500-SRW-Duramax-longbed-4X4
2008 Keystone EVEREST 348R 5th wheel
2002 SUNDOWNER gooseneck horse trailer