Dizzy - I agree with most of your points. Only thing I disagreed with was the original question from OP. Which is the better workout? As I mentioned above, with all things being equal, riding fixed is a better workout for the simple fact, you cant slack/coast.
I do not race. Just addicted to riding. I try to get around 150~200 miles in each week. To be fair, most of my friends ride the hills of PV on weekly basis so you can say we had home field advantage. We ride both fixed and road but we all believe fixed is a better work out. Im always impressed with people who can complete a century on a fixed.