I do both, kinda.
When I do my flights online (90% of my flights) I use VATSIM. I use FSINN, I actually find that more useful and user friendly than SQKBox... I'm just nuts
When I'm not flying online, I'm usually doing some other stuff, like web programming projects that are piling up on me. I find a nice long 6 hour flight is actually the perfect time to do work.
And then since I'm not using my VATSIM ID, I can always do some controlling as well.
But I use FSPAX (Christmas gift from my wonderful wife), like somebody said, it adds a whole dimension to flying.