Just about every swim cap made for competitive swimming is silicone or latex.
Your cap choice should depend on the kind of training you do, how often you train, and the pool temperature.
The Benefits of Silicone swim caps:
They are soft, and therefore better suited for swimmers who have a fair amount of hair on their melons. The cap won’t tug or rip at your head and hair. Because they don’t stretch very much, silicone caps also last a long time.
Silicone caps also don’t have as much surface area, and don’t wrinkle at the back and top of your head giving both a superior fit, and also a more hydrodynamic fit.
They are also generally considerably more expensive than latex caps.
The Benefits of Latex swim caps:
Latex caps are cheaper and more breathable than their silicone counter-parts.
Because they are thinner, they provide better ventilation of heat you’re your head while you are training your butt off in the pool.
We’ve all trained in that pool that is ten degrees warmer than it should be, leaving us visibly sweating and red-faced. Having a thick (silicone) cap on our heads only makes this over-heating worse.
On the other hand, they don’t last well when unattended. Let’s be honest here—there are more times than you can count that you’ve left your gear to rot at the bottom of your swim bag for a week or longer.