It also depends on what part of Texas you're coming from, it's a pretty big state (in case you hadn't yet noticed, hehe).
For example, I live near Dallas so the route I would take is quite different than the route someone may take from Houston or San Antonio. I choose to take what I call the Northern route thru Amarillo, Albuquerque, Flagstaff, etc as opposed to the Southern route thru El Paso, Tuscon, Phoenix, etc.
I have taken both routes, and find them to both be of comparable distance and time, however it's my opinion that the roads on the Northern route are FAR better than those on the Southern route. For that reason, we prefer the Northern route.
Another note, if you take the Northern route be sure to check weather in advance as the weather can drastically change very rapidly driving up the mountain to Flagstaff AZ.