If you don't have any previous experience using aerobars, we typically recommend the L-Bend because they put your wrists and arms in a more comfortable, ergonomic position. 

This is usually a better fit for triathletes and long-distance riders. If you're planning on using the aerobars for shorter, higher-intensity efforts like a time-trial, then the S-Bend shape can give you slightly more leverage to put out maximal power. 

Ultimately, it is up to your own preference.