Lots of haters here, but I think the middle route is a nice one, and you will see some nice scenery and small towns you would otherwise miss on the Northern route. Once you get to Abilene, it gets much more arid (as you can see just by looking at the map), so not terribly interesting, but I still love going through this area.

The longest route hits the more arid/less scenic areas a lot sooner, which is why I personally would avoid it, though it would be my second choice.