I have an Undercover one-piece on my truck. Pluses are security and can double for a work platform. Negative is having to either pop it off or ride with it partially open and bungeed in place when I load something taller than side rails of the bed. It pops of in about 60 seconds, not a big deal but a two man operation.
Having learned this the hard way, I will go with the Trifold next time - no need to remove it and pretty much water tight from what I have gathered from others. No more water seems to leak in than what comes in from around your tailgate with any other system.
"Sit down wrong, and you're beat." Jim Spencer