There is a large bump thread discussing this at

http://www.flyertalk.com/forum/showthread.php?t=278082

My rule of thumb is I avoid 50 seaters because they on average have only one or 2 overbooked seats. They are more likely to fill up on days when larger planes don't though.

I like to book the heavest travel days so I always go for the larger planes. When a flight is overbooked by 30 you have a much greater chance of a bump.