A good kevlar sail has a longer useful racing life if properly cared for. Dacron will lose it's shape quicker and especialy a headsail will be slower since they are fewer ways to adjust them.
True kevlar needs better care, rolling as has been mentioned and just as importantly keeping it out of the sun except when it is not in actual use. A sailmaker friend of mine leaves his main cover on until about 10 minutes before a race. If there is a significant delay between races he puts the head sail down below.
Kevlar shrinks in UV, if you have a 5 or so year old main it will likely be about 6 inches or more short on the hoist. All that being said if you want to race competatively you must use them.