I've driven both out to SoCal, and I prefer the 20-10 route. You can fly through TX - especially once the speed limit bumps up to 85 somewhere west of Midland-Odessa. The only thing 40 has going for it is a nice little stretch of Route 66 in Arizona. Other than that, it's a pretty crappy drive - especially once you realize you have to go through Needles, CA. Also, the drive up 287 to Amarillo beats me to death every time I drive it because it's such a bumpy-ass road. Lots of small towns and stops, but nothing to see.
So, yeah... 20-10 is my preference.