Hurricane12 pretty much hit the nail in the head.
Small unit leadership (USNA company level and subordinate) will be the level that Junior Officers will be working at when they arrive in the Fleet/FMF. The leadership/people skills you gain, while in this position at USNA, can really be beneficial and it is a great "trial and error" period in applying what has been learned over the course of your life (and 3 years at USNA). Hopefully you have some sense of what being a leader means and aren't a "poor leader" going into your 1/C year! Notice I used the word "can" because, as Hurricane mentioned, it depends how much YOU make of it. You can really be determined to see those under your charge succeed or you could be the person who doesn't really case....I highly recommend the former!
I never was anything except Midshipman In Ranks (MIR) within my company 1/C year (I didn't have a say
)....looking back on things, I would have wanted to be a squad leader in a heart beat!
As far as a Brigade/Regimental position, that can be a good experience as well. I did serve on a Brigade FUNCTIONAL position, didn't wear many stripes, but I was an active participant at what I did. I was more concerned about how I could help the Brigade in a certain area and implement good ideas. Brigade positions (normally 4 stripes and senior) require someone to really balance the Brigades needs, ideas, wants, etc. vs. what the Commandant and his/her staff decide and provide direction or course to steer. Additionally, there is normally very little company interaction and it can be hard to live the company life.
Some Brigade (and maybe Regimental) positions have separate nomination and selection processes, normally reserved to functional positions (i.e. Character, Honor, Physical Missions, MISLO, SARP/SAPR, etc. who go through a representative from those areas). However, most go through a board process, convened by the Commandant/Deputy Commandant.
To reiterate what Hurricane said, the goal isn't stripes....it should be HOW YOU want to make a DIFFERENCE!