The SF2 is more musical and less bloaty. That being said, the highs are the only thing that I am not crazy about with the SF amps. They seem to be a tad congested.
I do not remember about biasing the SF2's, but I remember it is no fun with the ARC. The other thing to be aware of with any ARC amplifier is when a tube goes, it generally takes out a resister and a foil. Unless you have a great and reasonable technician, ARC will gauge you.