I meant to suggest that interning for a judge, IMO, is more interesting than working for USAO. Any possible help for clerkships is simply an added benefit.
As a 1L, it is very unlikely you will land a judicial internship that will give you as much exposure as USAO.
What do you mean by "exposure"? Exposure to researching/writing? Exposure to other lawyers? Exposure to observing legal proceedings? Regardless of the definition, any one experience is likely to depend on the particular office; not all judges/USAOs are created equal. I interned for several judges and with a USAO. My experience has been as follows:
1. The researching/writing was different, though not necessarily "more exposed," at the USAO. As an intern for judges, I found that deadlines were farther away and generally looser. Thus, while interning for judges, I had ample time to perfect my research and writing as much as possible. I felt more confident that I fully researched the issues and reached the best result. I think my writing from the judicial internships was stronger because I had the time to meticulously organize the writing and choose my words with care. I also could take plenty of time to ensure citation accuracy. At the USAO, the work was very deadline-intensive. Thus, the research/writing was more labored. In the same amount of time at the USAO, I wrote a greater variety of documents compared to the judicial internship, but all of the documents were shorter and less detailed than the documents produced for the judicial internships. So, the exposure to legal research/writing is different. The USAO is probably more akin to private practice simply because of the deadline-intensive aspect of the work.
2. Regarding exposure to lawyers, I think there was generally more opportunity to meet lawyers at the USAO. At a USAO, you can go piggy back with other lawyers to every hearing and meet a lot of private practitioners (and public defenders if you are in the criminal division). This will, of course, take away from your other work. And because of the deadline-intensive work, I found myself often preferring to research/write rather than hang around the courthouse. As a judicial intern, you don't hobnob with private practitioners. But depending on the office, you may meet quite a few other law clerks and/or judges. At the USAO, however, you will likely end up working directly with a greater number of lawyers compared to interning for a judge.
3. I definitely had more opportunity to observe legal proceedings while interning for the trial judge compared to the USAO. Not so much with the appellate judge. But again, I chose to not go to the courthouse very often while working for the USAO; so this might depend on your personal preference. I enjoyed the researching/writing more.