It seems to me, admittedly without empirical information, that it would be entirely dependent on your approach. If you were to keep an evenly steady cadence on one gear on a bike with a freewheel then I imagine that the difference would be negligible. I think that where fixed really shines for a workout is that it takes away the option for laziness. Feeling a little worn out and a hill's coming up? Too bad. No lower gear for you. Just sprinted until your heart's about to burst? Sorry, we're fresh out of coasting. Catch your breath when you get home. Plus, the sheer fun of it will keep you on your bike longer and more often. All in all I would say that yes, fixed does provide a better workout, but not necessarily for the reasons that you might think.