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As with these things, its always a matter of taste, but I'd probably agree with you and take I-30 and I-40. Memphis and Nashville should both offer plenty to do, if they end up being places you have time to spend the night, and of course that route will take you through a little more mountainous country.

I-20 could be fun too, though, and I think you could really find some fun southern culture on that path.

Time wise, I-20 might be a little bit faster, but I doubt it would be by much. Its probably a wash when you factor in the possibility of being slowed down by Atlanta traffic, so I would make my decision based solely on which route you think sounds more interesting.