How were you able to decide what aircraft/mission was right for you when it came time for aircraft selection?

It will be the one opportunity that you get the most amount of say in your career in the Army. Study hard so you hopefully get your pick. I say "hopefully" because, at the end of the day, it's still Needs of the Army that'll determine how many, and of what type of aircraft are available. But, it's still worth the opportunity to determine the rest of your career.

Did I get as much time to go to Destin as some of my friends because I was studying extra on the weekends? Nope, but I got my first pick of aircraft because I put in a little extra effort, and I wouldn't change a thing.

The needs of the Corps decided my initial assignment, however, as time went on and conditions changed, I had the opportunity to serve in all types of aircraft and missions. Stay long enough and I'm sure you will have all types of experiences if you pursue them and sense needs and opportunities as they evolve.

As guard I didn't have to select like you do but I did get a choice between Blackhawk and Chinook and choose Chinook. I picked Chinook partially because it gave me a type rating, (Hawks might have one now but didn't then) and my BS in Aviation that I was almost done with required either a CFI or a type rating to graduate.
Inexplicably the Guard and Reserve students still had to attend selection day. Blackhawk's had the most duty locations which I think is a valid reason to choose hawks. Apaches allow you to blow stuff up which does seem pretty cool and something you'd only get to do in the military, and the almighty Chinook clearly caries the most, including a few more comforts when going to the field. I think you guys get a few fixed wing slots too these days. That wouldn't have interested me at time as I specifically sought out the Army for helicopter flying.
When I went though almost entire classes were getting either Blackhawks or Apaches which invariably left more than a few students upset as it seemed like the two camps were lift or guns. If you don't get what you want try to be happy that you're getting paid to fly and then fully buy in on whatever you're assigned.