I'm shocked that this question hasn't been asked already, but I swear I looked everywhere. Are there advantages of thin over unicorn, or vice versa, when running Rails 3 in development mode?
I decided to go with Thin for development because Heroku runs my apps on Thin in production.
It's always a good practice to make your dev ENV match your production ENV as much as possible.
Here's a couple breakdowns of Thin vs. Unicorn, although they're a little old.
http://snaprails.tumblr.com/post/441654760/thin-vs-unicorn-performance-benchmark