PRIME is an über-expensive steakery, as joints of that ilk tend to be. Our first time there years ago we vowed never to go back owing to the airs and what was delivered overall for the price. Hey, every joint messes something up some time. A client took me back, and it was generally improved -- the meat has been w/o fault both times. They paid. I'd still not.
MESA is cookie-cutter, and that's not necessarily a slam. Let's go with formulaic. We've eaten there more often. The first time was a flop, and that was right after opening. There have been no problems since. You have more diversity on the menu and the environment is lively and more vibrant-chill. (On a personal level, SW is a staple in our kitchen, and we cook all the time from this joint's cookbook.)
Totally different animals, vibes, styles, eats, experiences.