I've moved by 2's so far. 15-17-19 and I've found that each change has been fairly significant, as in if I did a smaller change I may not really feel a difference. On my Jamis I have a 19/17 Fixed/Fixed rear hub. I ride 19 on my commute, and most night rides, I find it's actually a bit faster then my 17 for most practical riding, I have a tendency to beat a lot of people to the top of hills without a standing effort, it's only on the straight away sprints that I can really feel the spinny-ness of the 19t. I take that as my need to increase my cadence, rather then my gearing(also my stamina.) Over all i'd say go up to 19 if you have a lot of rolling hills, I seriously enjoy riding a lot more when I can take hills, up and down without destroying myself on each one.