For picking an origin character (from my limited experience of ACT 1)
Pros
You get a party full of back stories, custom events and special dialogue. If you choose a main custom character, you still get a lot of this by picking up origin characters to join you.
Cons
If you don't like the style/bonuses of the origin then you can customise your own character completely. They will still form relationships, and be more of a factor of their tags. I chose this option and am loving it (I weave my own story/character through the options etc).
It really doesn't matter so much - choose something you will enjoy!
