you can't say fixed is a better workout, just the same as you can't say that spinning classes are better or worse than road workouts. you can be lazy on a fix, you can be lazy with gears, that's not what matters.

what matters is that you're working hard enough. fix is good for aerobic workouts, since it forces you to keep spinning, but you need to have a heavy enough gear and a fast enough cadence to keep your heart rate up. fix can also be good for hills/strength, but then you have to have a really heavy gear or some good hills to make it work.

bottom line: you can get a good workout on any bike if you're trying.