I know several people with 310s and they have been great saws for them through out the years. One guy was gifted one from his grandfather, tore it down to clean it up and got it back together and is running it. When I started working at a dealer, I really liked the 311, but found that the 261 did a same/better job at less weight. I also have two 271s (both rebuilt project saws) and they run great too.
I don't doubt the CS 6700 will run circles around these. I agree about the built in flywheel key, I have taken several saws apart and then panic and start looking for the Woodruff key that fell out only to find out its built into the flywheel.