I don't know the exact details since I only went this year for the first time, but it seems like theres a "best of theme" trophy for all themes. So if you're building your city out of mainly Bionicle, city, or ninjago (just for example) you have a chance at winning the best of Bionicle, city, or ninjago trophy. I don't know the exact details but I think the winners are voted on by attendees (someone please verify all this for me).
I have the link to the Brickfair website for any important registration info you may need.
If you're planning on showcasing your MOCs, you'll be at the expo center before it opens up to the public, and thats when you want to go around buying pieces. The vendors run out fast. Most vendors don't even sell pieces in bulk. Most I saw sold rare/exclusive minifigs, custom minifig parts, or sets. You can get some good sets there but you have to be quick.
If/when you go, I hope you'll enjoy it. It was a ton of fun just to walk around and see everyone's MOCs and talk to everyone. I highly recommend going at least once.