Winter training, geared or SS?

Combined with some turbo and stretching I'm pretty confident things are going better with gears. Geared you can choose how hard you work, and when, and you can ride with normal people ;-)

This is pretty much how I see it. Fixed is great for commuting (and short/relatively flat rides), but the gear I'd use for commuting is a bit lower than the one I'd use for 'training' - which either means compromising or fiddling around changing ratios (even with a flipflop it's not straightforward). The last couple of years I've just used a relatively cheap alu road bike all winter (and summer too) and feel much fitter/stronger when the spring arrives. Which makes sense - if your aim is to be faster on a road bike, then just ride a road bike pretty much all the time (although obviously less hard in winter). I've gotten out of practice going up hills properly when I've ridden fixed for too long, and it takes a while to get this ability back. I just have the odd fixed ride every now and then to keep things varied and fun.