I would say that I-10 is the most dramatic scenic change interstate high way from coast to coast in the States. Along I-10 from Florida coast to CA coast, you will see trees change from pines to a kind of very tall trees in Alabama, a very special trees in the wet lands in Louisiana, to short oaks in east Texas, to small bushes in the west Texas, desert in New Mexico, saguaro in Arizona and back to pines in CA coast.
Most important, you will pass many places that deserve you stop to see. Florida pane handle, you will see the snow white beaches in Pensacola; an air carrier just by I-10 in Mobile, AL; New Orleans, Louisiana, though destroyed by Katrina, I think French court should be okay since they already held Mardigras last February; Houston, you can see NASA, don’t forget have a good dinner in China town; San Antonio, most Spanish taste city in US; White Sands in New Mexico; Saguaro in Tucson, Arizona; then you can either continue I-10 pass the pink city Phoenix to LA or switch to I-8 to San Diego, the most beautiful big city in US.
“Iliketravelling” in this board just drove to New Mexico and Texas and posted a very good series of articles. I recommend you to read them so you have some senses of part of the sceneries on your road.
If you choose I-20, you can pass Dallas, a very boring metro. But you can pass Midland, the home town of G. W. Bush if you are one of his fans.
But keep in mind that you need to have a robust AC in your car. I don’t think that you can survive driving I-10 without AC. When I was in Austin, TX, many AC of my friends’ car (including mine) got broken since they had been working too hard.
Enjoy your trip!