As usual, Jacy speaks the truth.
Although I usually advocate the clerkship route, there may be a caveat here if you can't do both. I assume the judge is a state judge, correct? If you can only do one, the RA position is the right choice if, and only if, the prof gives you some kind of assurances that (assuming you do a good job) he will go to bat for you with his friends in high, highly paid places at firms. If he is "well known" and therefore well connected, having him as your champion may be worth the haircut you'll take on substantive summer experience.