I have done both, here are my thoughts. As far as future employement, they are just about equal. Many firms -- even small ones -- will keep outstanding clerks as lawyers. If they don't, they are great contacts and will sometimes help you find work. Judges often know a lot of people and can also help you find work.
I, personally, learned more about practicing law working in my clerkships than working for a judge. If you need the money right now, I would lean towards mid-level firm. UNLESS, one of the judicial internships is with a Federal Judge.