This question was posed to me by one rider a long time ago. I still remember how he asked it - if you are given a choice in the velodrome, would you choose time trial to pace by yourself or the common pacing provided by some other riders? The question was actually which would give you a faster time. Although I still have to try it myself but I guess pacing is better for it somehow elicits more power from the cyclist unlike the time trial where you are challenging only yourself.
I have never ridden on a velodrome, but I have ridden on a track at one of the local high schools here. While this track is flat all the way around, you still get the feeling of being in a race if you are riding with other cyclists. I have done both time trial and pacing on this track and I honestly prefer pacing. I just feel this is easier for me because I have better control of my speed and how much I exert myself. When I was a teenager I loved a good time trial race, but now i have to base the way I ride on how severe my fibromyalgia pain is each day.
