I don't think it will make a huge difference, but I think you're better off with the judicial internship. Working with a judge is always a great experience to have on your resume, no matter what type of practice you want to get into. Employers know that a lot of people do the RA thing when they don't have other options for 1L summer, so the judicial internship might give you a slight edge for next summer. However, if the prof is working on something really cutting-edge or simply something that you're deeply interested in, go for it!
Oh, one other thing- check to see if the RA is possible during the school year (in which case, you can get both experiences). A lot of people at my school do that for pay or course credit.