Tent vs bivvy bag - different camping experience, protection from the elements, comfort and weight? What are the pros and cons of both? Is it even a valid comparison - e.g. is it viable to rely on multi-day camping if you only have a bivvy bag or on the contrary does it make it easier to find somewhere to sleep?
Tents, tarps, and bivvy sacks are three different specialized tools for three different jobs.
I typically prefer a tarp to either a tent or a bivvy sack, but that's because I do most of my camping in California in the summer, when the weather tends to be quite dry. In summer in the Sierra, I usually bring a tarp but sleep in the open and never take the tarp out of my pack. A tarp is easiest to set up if you have trees, and works best if there is no wind. Setting up a tarp takes some skill and practice. In a wind, it's hard to keep a tarp from flapping noisily and keeping you awake. The great thing about tarps is that they can be so compact and light. My tarp is 6.5 oz (180 g).
If I know that a trip is going to involve snow camping, I'd prefer a tent. Some people prefer tents to tarps for summer camping because they keep bugs out, but what works well for me in mosquito season is to sleep in a headnet until the air gets cold enough that the bugs become inactive. Expensive ultralight summer tents can be almost as light and compact as a tarp.
If I'm planning to do a climb in snow in a single day, but want a safety net in case I need to camp, I might bring a bivvy sack as a more compact and lightweight alternative to bringing a tent that I don't expect to use. I have one and have brought it as an emergency tool, but have never slept in it. I have talked to quite a few people who have used bivvy sacks, and none of them ever had anything good to say about spending a night in one. Everybody complains about condensation building up inside the sack, leaving you sleeping in a pool of water. Bivvy sacks vary a lot in weight. Mine is 8.5 oz (240 g), but I've seen others that are as much as 39 oz (1100 g), which seems absurd to me since you could get a tent that weighs the same amount.
For more about bivvying a must read is "The Book of the Bivvy" by Ronald Turnbull. For more about being close to nature read "The Wild Places" by Robert Macfarlane.
I have run into this HYPOTHETICAL dilemma with bivvy sacs: I have long liked the idea of a little cocoon to just snap to a couple points and presto sleep.
Each time I camp, I look around either while pitching my tent, or in the A.M. when the sun comes up if I have to, just to take in the location, and maybe consider a few what-ifs.
I have often seen places where there is not a lot of practicality for bivvy sacs due to the lack of natural fixtures to append them to. What I mean is; it sure sounds cool in theory to string your hammock or bivvy sac to a set of trees, but in practice that is not always possible.
Consider your terrain, know it well, know your overnight spots and ask people on forums who may have been there if you can.
I am considering a Pacific Crest Trail run to Oregon this summer and I really want to take a bivvy sac, but I don't think I can reliably guarantee a set of trees to sling it on at all times. Sometimes you just gotta crash out, and the darkness has set in, you become dangerously tired.. that trail is too narrow and a mess to just improvise outside a designated overnight spot..
I have seen this in a few spots along the PCT in the North Cascades, and what I came away thinking is it would be real inconvenient if I had brought a bivvy sac.
That being said, I obviously am not a seasoned bivvy sac user; so take my observations lightly with a grain of salt. However; that is my take on the bivvy sac dilemma.