If it were me, I'd take I-74 from Indy over to Cincy, then take I-75 from there down. I-65 just goes so much farther west than I-75, that the slight southwestern tack 75 takes from Knoxville to Chattanooga doesn't compare to it, time-wise. Taking I-74 to Cincy from Indy, at least has you on a generally southeastern track during that leg, so you're not going out of your way to get to I-75 there. I-24 is a very nice road, I'll admit, but this will take more time to get you over to I-75 than taking I-74 would in Indiana. The only reason that one map may have mentioned I-24 as being faster than I-74 is that the speed limit in Tennessee is 70, rather than 65. However, I can't imagine that would make that much difference.
I'm unfortunately VERY experienced with I-75 in Kentucky. About the only really crowded spots in Kentucky are just south of Cincy, and in some areas near Lexington. After that it doesn't really get too bad until Knoxville, which, no offense to Tennesseans on here, seems to have ENDLESS construction going on on I-75. The traffic isn't terrible, but the endless concrete barriers start to wear on you a little, after a while.
Do NOT take the bypass around Atlanta. Go straight through, and if you have more than 2 in your vehicle, get in the HOV lane there. Likewise in Macon, avoid the "bypass" there.
Have fun planning!