My reasons for preferring LA are purely personal and #theweatheristoodamncold, and I'm optimistic that I can make it to either location this year after several years away.
But the most valuable sessions for me at C4LConf for me have been the BOF/breakout sessions. This is very different from multitrack presentations. Breaking into small groups is often really valuable; breakout *presentations* rarely so. In a smaller group, people feel more comfortable asking stupid questions, which are usually the questions everyone else has.