Poncho tarp: pros: light, provides rain gear and pack cover for same weight. Cons: least protection, needs bivy (added weight and cost), lots of strings and stakes.
Gatewood Cape: lightest (no bivy weight), provides rain gear and pack cover for same weight, 360 degree protection, easy pitch (6 stakes, one pole, one guy line).
Flat tarp: pros: many pitches possible, very "universal" gear, larger size (8x10) can be used for 2 people. Cons: not as much protection as GW Cape, smaller sizes (5x8, 6x8) need a bivy for full protection, lots of strings and stakes, no rain gear/pack cover options.
If it is just a couple nights a year, you can probably find a lighter poly tarp. Or wait like a spider for a deal on a tarp.