Fixed-Gear vs Freewheel
My winter bike is clunky, slow, and single-speed. For a couple of years I had a fixed gear on the back, but that wore out, and I ended up replacing it with a freewheel limited to a single-speed. The transition has been educational.
Fixed-Gear Thoughts
Fixed-gear bicycles have some definite advantages:
Because you brake with your legs, you need not have your hands at the brake levers at all times. This is a big advantage when it is cold.
Braking and slowing down with your legs works out muscles that are otherwise not exercised while biking.
Riding mild inclines becomes somewhat enjoyable.
Fixed-gears work in more weather conditions than freewheels.
However, there are significant disadvantages:
Fixed gears are difficult to find. I had mine custom-made, and now that it is worn out it is tough to get another.
Going down hills is stressful. You cannot coast on a fixed-gear bike, so if you cycle too quickly down a big hill your feet can easily slip off the pedals, resulting in fear and pain as the pedals whack your shins all the way down the hill. Thus I tended to take downhills slowly and carefully.
Mounting and dismounting the bike became a pain. The best position to start is with my left foot high and my right foot close to the ground, but this is a difficult position to stop in. Sometimes I almost fell off the bike attempting to do so.
Freewheel Thoughts
Freewheels also have advantages:
They are readily available, especially since Recycle Cycles exists. Modifying a freewheel for single-speed use requires a vice and chainwhips, but it can be done on many freewheels.
Going down hills becomes manageable again. Being able to coast makes a big difference.
To reduce the number of moving parts I removed the derailleur from my bike. It may be possible to add it again using a freewheel system. The derailleur would not help me change gears, but it would maintain tension on the chain, which is a really irritating problem I have with any kind of single-speed system -- maintaining proper chain tension is a big headache.
And they have corresponding disadvantages:
They can stop working in the winter. In particular, the insides can freeze up with ice. In the worst case this means the internal teeth lose their grip, and the freewheel spins freely in both directions! This is the problem that made me turn to a fixed-gear solution in the first place.
Braking with hand brakes sucks in the winter. Not only is it cold on the hands, but if it is wet and slushy outside your brakes do not work as well.
Conclusions
I have no sweeping conclusions. Neither system is great but both can work okay. Winter cycling is a pain no matter what you do.