Had a Peragon. It was fairly tight (except at the tailgate) and then I put some rubber trim there, and it was just about 100% water tight. Big annoyance was making sure everything was low enough to fit under it. Got a dog, and had to move to a Leer cap. They aint cheap, but they are VERY convenient. Looks great (like it's part of the truck), locks and is dry as a bone. Crate for dog fits no problem. Annoyance of cap is you can't just tie down the back window when you have long stuff (Leer says that's the #1 way to break it).
Other option for a top is the bakflip. Friend had one and he used to load things (like christmas trees) ON TOP of the cover - I saw him standing on it. When it is folded completley up, the bed is almost completely uncovered, but the back window is blocked 100%. If you are used to driving a large truck, this is no problem, since you are used to using the side mirrors for everything.