Normal tracks (21 cm, lower picture): Good for fast skiing; diagonal skiing, "double kick poling" and down hills.
Narrow tracks (17 cm, upper picture): Good for very steep uphills (more running, than diagonal) and double poling.
We also tested the tracks skiing very slowly. In general we think the narrow tracks are better for slow skiers, i.e. people skiing at low pace and frequency, while the broad tracks most of the time are better for competetive skiers. The best solution will probabley be to change between the two, depending on the terrain formation. I.e. For slow skiers: broad in the downhills and narrow for the rest. Competetive skiers: narrow were flat and in the very steep uphills, broad for the rest.