Weighing in here: My main problem is that my hiking companion is an 80-lb. dog. I therefore require a bug-free shelter for him as well as me! In addition, he feels more secure and comfortable in an enclosed space that reminds him of his crate at home. For these reasons, I take a tent. It's a really lightweight tent, the Tarptent/Gossamer Gear Squall Classic, 27 oz. with stakes. From the research I've done, a tarp for two, groundsheet for two plus bug net for two (haven't found any of the last so I'd have to figure out my own) would weigh just as much. The tent goes up really fast and is very comfortable. I can have it up within a couple minutes of stopping for the night. I can see out the back and front, and even the sides if I put my cheek on the ground instead of on the pillow atop my 2.5" thick insulated air pad.

I really would like the freedom of view of a tarp, but not during bug season. They don't appear to make bug nets for dogs, lol! I plan to try it next fall and see how Hysson reacts. If I have to keep him tied up, it won't work. Every couple of hours he stands up, stretches and then turns around three times before lying down. If I have to tie him to keep him under the tarp, I'll be spending the night untangling him.

Re a stand-alone bivy: It weighs as much as a lightweight tent and there's no room for my dog. Plus I have to get up several times a night (old age) and, when it's raining, appreciate the roof overhead so I can crawl out of and into my sleeping bag without getting wet!

Your Mileage, of course, May Vary, especially if you don't have a relatively large dog to fuss over! Of course I very much enjoy fussing over him--he's my best hiking buddy!