I ride 46x16 and used to ride 46x18 (and I still have the 18t cog on the other side of my hub). They are appreciably different (almost 8 gear inches I think). I used the 18t cog when I lived in a hilly-er area when I first started to ride fixed. It worked out pretty well, although now I just stick with 16 for the most part, even when I'm in hilly areas.