I currently throw 15lbs but I also have some 14lbs that I have thrown on occasion. I would like to drop down to 14 since I get more ball speed and can put more on it when thrown correctly. Sometimes I do tend to pull the 14lber early in my backswing leading to a timing problem, but I'm sure that would be fixed if I went to 14 on a regular basis.
When thrown correctly, I don't notice any difference between a 14lber and a 15lber especially from outside lines. However, if I miss it at the bottom (which I do on a regular basis) the 15lb tends to carry better.
At my skill level, I tend to score better overall with 15lbs. If my ball speed drops to the point where I lose carry, then I would switch to 14.