Hi, It is a good question. And you answered it yourself. It is mostly personal preference...and a bit of culture too.
For example I say Hey, how's it going? How are things? How are ya? Very commonly. I would use them when I see someone in the street that I know and am pretty good friends with.
You could also use them over the phone too.
If someone says "How are you?" They are asking about you specifically..."I'm fine" would be a good responce.
If someone asks you "How are things?" or How are things going?" they are asking more about your life (work, family, friends, etc). So you might answer... Things are ok...but work is killing me. Or you might say...Not too bad...etc.
I think the best thing is to choose a couple that you are comfortable with and use those...and forget about the rest. You don't need to have 10 different greetings...people always use the same ones.
I hope that helps.
Student or Learner