A lot depends on what you are wanting to see. The I 10 route will be the most direct. Understand that it will be a 10 hr drive just to get through Texas. You will get to see a lot of desert like scenery along the way.
If I were inclined to go by way of Dallas, I would use US 287 north through Wichita Falls on to Amarillo and pick up I 40 there, unless you just want to go to OK City. If you go this way to Vegas, you can expect to add about 5 hrs to the entire drive. The biggest thing about the I 40 route is much of it is along the old Route 66 corridor, so there are many little curio shops along the way.
A good GPS will help with the navigation, and is a whole lot more accurate than mapquest.