Well, you literally choose one or the other when you are kid for whatever reason you have. In my case, my folks went to Alabama and we had season tickets. I had a stuffed elephant in my crib when I was a baby, and I do not remember a time when I was not an Alabama fan. I listened to my first game on the radio when I was 6 (1970 Iron Bowl loss to Auburn), and I went to my first game when I was 7 years old (1971 win vs. Southern Miss with Ray Guy!) and LOVED IT. However, my paternal Grandfather went to Auburn Vet School, which clouded the judgement of my older brother and my younger sister. All of us graduated from college, but none of us went to Alabama or Auburn. My brother and sister went to private schools without a football team.
As for the fans in general, the dynamic is like in Iowa with Alabama fans being more like Iowa fans (entitled, arrogant, etc.), and Auburn fans being more like Iowa State fans (more closely affiliated with the school for a larger percentage of the fan base and FOREVER TRUE.). There are more bandwagon Alabama fans than bandwagon Auburn fans just like there are more bandwagon Hawkeye fans than bandwagon Cyclone fans. All I can say for myself is that while I was born and raised an Alabama fan when Bryant was the Head Coach, I was searching for something else.
As fate would have it, I met a cute girl at a High School Game who had moved from Iowa, and I asked if she was an Iowa or an Iowa State fan. She said: Go Cyclones! I have been an ISU fan ever since. We went to the school dance after the game (every time I hear Hot Blooded by Foreigner I think of her). Then, that season (1978), Iowa State played in the Hall of Fame Classic @ Legion Field, so I went to the game, rooted for ISU and have followed them ever since that time.
Fast forward in life, and I landed at Iowa State after serving in the Army even though I had only been in state twice and had never been to campus. Had another brother living in Des Moines at the time, so I applied to ISU, and the rest is history. Best decision I ever made. I love Iowa State University with every fiber in my being, and I am an Alabama football fan out of habit. A bad habit. Like being hooked on heroin bad. I try to break away, and the fans get on my nerves if I talk to one for more than 5 minutes, but I still like watching them play. It all gets back to the stuffed elephant as a kid.
As for Alabama and Auburn fans in general, they are ALL CRAZY! They take football way too seriously. The ones who attended either school are more balanced than the ones who grew up as fans of one or the other but didn't go to college. Auburn fans are generally nice people UNLESS you talk to them about Alabama. Then, they get all riled up. That game is literally the most important game to them (especially @ Jorrdan-Hare). They use VooDoo dolls and cast spells on the Tide. Does not matter how good Alabama is, they struggle @ Auburn. Meanwhile, Alabama fans just don't want to lose to Auburn and mess up a Championship season. Tide fans also love to talk about how many championships "THEY" have won. They really personalize that more than most. One of the reasons, I do not say "WE" when talking about ISU, even though I am a graduate.
In general, they are both fantastic places to go see a game. If you have not been, I woudl suggest going to a game at both places and have a good time.
Hope that helps. I know I rambled a bit.
Let's Go State! and Roll Tide!