I have had some experience with all types of setups. It always comes down to expected conditions. A tent or bug tent is necessary if you expect swarms of mosquitoes. Otherwise I much prefer a tarp over a tent for freedom of movement, storage space, and ventilation. If you take only a bivvy and it rains you definitely need a tarp because you would otherwise be trapped in the bivy bag and can't cook, sort gear, change clothing, pee, read or do anything without getting wet. So if there is [/b] any chance of rain[b] take a tarp with the bivvy. I used a Mylar"space" blanket for a ground cloth last time I was out. It got a few small holes in it after 2 nights, but worked... sort of. A lightweight plastic painter's tarp is more durable.
If you are inexperienced it would pay to take a little extra gear but see what you can get by with on a couple of weekend trips.