Here's my advice in 16 years of doing this. When I started I always wanted to be a referee (don't know why) and ended up being one for 8 years until promoted to work in college. If you want to be a U then definitely try to work U if you can and as a second position take either wing. Do you think you will have aspirations for college? If so then I'd work wing and a back judge. Here's a quick synopsis of my career for all it's worth.
Worked as high school LJ, hence my screen name. I worked LJ for only 2 years then moved to R. After working 7 years I moved up to NCAA. Although that was a nice move to work R it didn't quite prepare me for college (becoming a college R the choices are limited and I was already in my 40's). When I got to NCAA I applied to F knowing they needed deep guys. Having LJ experience it still helped me for working sideline so had those core competencies. I worked F for 6 years in college and was promoted to B last year and now working D-1 (FCS). The F helped me for learning this position and my boss knew I had what it took to work deep having worked D-II playoffs as an F. Had some speed and excellent rules knowledge. With "deep experience" I have given myself skills to work 3 positons (F, S, B). If you want to be an R then I'd recommend for you work L (or H) and B. A lot of B's become referees.
If you think you might want to move to college (or if you want to work BJ anyway) I highly recommend you ask for H or L for the one positon, then B for your second position. Absolutely. You well be well-groomed to work the wing and if you ever want to move up to college (where they also have a field and side judge as I mentioned) you will have the skills necessary to work deep as I stated.
I wish you the very best of luck.