Cassin's Kingbird?

Here's another bird I saw on my Las Vegas trip that I am uncertain as to its identification. I can't decide if this is a Cassin's or a Western Kingbird. I tend to lean towards the Cassin's because of the defined white stripe on its throat and extending under its eye, even though there seemed to be a fairly large variation in this feature for both Cassin's and Westerns in pictures I found on the web. It's breast also seems to be a bit dark for a Western, but haven't seen a Western Kingbird since I was a kid.
Again, any feedback would be appreciated.