Silicone lubes are slick and won't need to be reapplied too often. They also are less likely to cause an allergic reation to sensitive skin. If you're planing on using lube in water, silicone would be your choice because it won't break down like a water based lube would.
If you're using a silicone toy (which most are) DO NOT use a silicone lubricant. The silicone on silicone produces a reaction that will cause the toy to deteriorate.