I used a Gore-Tex bivi for years when I was in the army, in Britain, Germany, Norway, Kenya, Belize, the Falklands etc. I also used it while alpine climbing on multi day climbs, sleeping on a ledge with your feet dangling over the edge of a 1000ft high rock wall does take some getting used to. My old bivi is just a simple sleeping bag cover, no hoops or other fancy refinements. Getting in and out of it in a rain storm is not the easiest of tasks and it does require a certain amount of agility, something I'm rapidly loosing.
I brought it from Survival aids 43 years ago, it cost me £50, it weighs 500g and rolls up to approximately 25cm x 10cm diameter. I've two "Backpacking" tents, one weighs 700g but it is a trekking pole tent so when bikepacking needs the extra poles adding 145g. This rolls up to 40cm x 10cm diameter plus the pole which are made up of three 40cm sections there are two poles. The other is a little heavier at 1200g but is bigger internally, it rolls up to 40cm x 15cm diameter, including the pole. So really I wouldn't gain much space by using a bivi over either tents.
To me the main disadvantage of the bivi is all you can do is lay in it. In bad weather you still have to get out, you can't sit out a storm or even a shower. You can't cook in it, you can't even get dressed in it. The inner tent of both tents are mostly mesh which keeps the midges out. No such protection in the bivi. So I suppose it all comes down to where and when and how much you value weight saving over comfort.
I brought it from Survival aids 43 years ago, it cost me £50, it weighs 500g and rolls up to approximately 25cm x 10cm diameter. I've two "Backpacking" tents, one weighs 700g but it is a trekking pole tent so when bikepacking needs the extra poles adding 145g. This rolls up to 40cm x 10cm diameter plus the pole which are made up of three 40cm sections there are two poles. The other is a little heavier at 1200g but is bigger internally, it rolls up to 40cm x 15cm diameter, including the pole. So really I wouldn't gain much space by using a bivi over either tents.
To me the main disadvantage of the bivi is all you can do is lay in it. In bad weather you still have to get out, you can't sit out a storm or even a shower. You can't cook in it, you can't even get dressed in it. The inner tent of both tents are mostly mesh which keeps the midges out. No such protection in the bivi. So I suppose it all comes down to where and when and how much you value weight saving over comfort.
