"different"
When you say "better workout," I have to ask "for what?" It is different. I've ridden exclusively fixed gear for up to two years at a time, and in some ways I got much stronger, and some weaker.
Fixed gear of course tests your extremes. Just this last Saturday, I rode my Pista with about a 70" gear up a huge hill, 2,800' in 6.5 miles, with sections up to 16% grade. Have to stand nearly the entire way, and anything over 10% starts to require extraordinary measures just to move, like pulling and pushing so hard on the handlebars that my hands, arms, and shoulders hurt for two days. Then, there is a corresponding descent at 25-30 mph spinning my butt off. It really beats you up, but what I've found is that there is actually very little threshold kind of riding -- seems you're either maxed or not riding that hard, as you have to rest up just a bit to make it up the really hard parts. On top of that, if you gear so that you can even get up the big hills, you'll be spinning out much over 20-22 mph on the flats. If you might time trial at 25 mph, you're not working that hard.
Commuting is somewhat similar. If you gear so that you're not destroying yourself on all the starts at intersections, you probably won't be working that hard while cruising.
Bottom line, fixed gear is more like weight lifting on the hills and starts, but less strenuous cruising at speed. You'll likely gain strength and lose a bit of threshold capacity. This has been my experience.
Best would be to do some of both.