Going through Montgomery is a shorter distance and seems a little faster. I don't think there is much time difference unless there is an accident or construction on one of the routes. Traffic is heavy around Atlanta and on I-75, but hooking up with 231 out of Montgomery is time consuming too. I know another person who has traveled these routes more than I have and he insists that going through Montgomery is faster.
There is a slightly higher percentage of interstate travel when comparing the route through Atlanta to the one through Montgomery, but much of Hwy 231 is 65 mph speed limit, so the shorter overall distance makes up for the slower speed. Birmingham to Montgomery is interstate.
Hwy 78 through Mississippi is very quick; some small towns and traffic from about 45 miles out of Birmingham.
On our very 1st trip to WDW, we drove to Tuscaloosa to attend a basketball game, then drove through Montgomery and that's not a bad route either.