Hi Michael,
Exits are needed most.[1] But if you want to run an Exit be prepared to deal with complaints. That link should answer your question in depth although the stats are four years old. It also includes tips to make sure you run an Exit with minor issues. Although the stats are outdated and the percentage of Exits have increased over the years, as it states in the post it is still one of the scarcest nodes. Hopefully others will be able to provide more information concerning updated statistics. But I believe it was covered at 30C3 last-year. [2]
Regards,
Jason Odoom
[1] https://blog.torproject.org/blog/tips-running-exit-node-minimal-harassment [2] http://youtube.com/watch?v=CJNxbpbHA-I
Sent from my Android, please excuse my brevity and any typos.