Could it be a bull snake? The pattern looks a little like it...
It could also be c. atrox. The pattern doesn't really look like it, but then again I havent seen that many western DB rattlers in my life. What did you mean by colour on its face? Atrox can have stripes on their faces:
I'm pretty sure its not a mohave... doesn't look right to be.
It looks like a rattler to me... but there are so many species in w. texas and without a better pic of the head it's pretty hard to be sure.