If you don't cap the T and it's of a decent lenght nothing will happen (the water is pumped around, not pressurized), but hey it's nice to cap it in case you turn your computer over.
I like the T's as they have less chance of getting air in the system once it's out. But the resevoir is much easier to fill. It's personel preferance. Do try to fit the T or resevoir to the highest point in the system (as this is where the bubbles will collect when it's not running).