I have two shelter setups right now: poncho-tarp + MYOG bivy and a Golite SL-3, recently acquired, + ...something (groundsheet or bivy, maybe even the matching nest).
Poncho-tarp/bivy: Poncho is 7 oz, bivy is at 6.5, so add in stakes and guylines and I'm looking at probably 18 oz total for rain gear, very good shelter coverage, ground cloth, pack cover, total bug protection, and a nice little bump to my sleep system's rating. I still use a mummy bag, but I plan on making a synthetic quilt sooner or later, so the bivy will help that as well by cutting drafts.
SL-3: Fly is 23 oz if I recall but sheds rain and snow beautifully (so I hear), and it is spacious, even for two people. Better protection from bad weather as well, no matter what is said about a tarp/bivy combo. The downside comes when you consider that I need carry a pole extender, still have to add in stakes, add some sort of ground cover, and the stock nest is ridiculously heavy due to the very tough floor material (because of the center pole trying to stab through it). I also still need rain gear, especially if the weather is bad enough that I'm bringing the SL-3. So carrying the SL-3 adds at LEAST 2 lbs to my pack.
To the bivy point: I wouldn't want to camp in bad weather under a poncho tarp without a bivy, and I wouldn't want a tarp versus UL tents unless it really reduced my pack weight, which a poncho-tarp does. So a bivy gets factored into the weight cost and the tarp/bivy combo still comes out ahead.