If you have much condensation in a bivi it's because you don't use it right, you should not be breathing into it and a little air should be able to flow.
Goretex is breathable but it's not magic if you sweat a lot or worse breath into it without letting the air vent it'll build condensation.
A tarp is no way lighter than a bivi unless it's a tiny thin tarp and a heavy thick bivi. They won't pack to much different sizes either.
For me it's no contest to bivi because you don't have to do anything it's always ready. A tarp you got to rig it up and at the end of a day it's a pain and you just end up rolling up in it using it as a second rate bivi.
Goretex is breathable but it's not magic if you sweat a lot or worse breath into it without letting the air vent it'll build condensation.
A tarp is no way lighter than a bivi unless it's a tiny thin tarp and a heavy thick bivi. They won't pack to much different sizes either.
For me it's no contest to bivi because you don't have to do anything it's always ready. A tarp you got to rig it up and at the end of a day it's a pain and you just end up rolling up in it using it as a second rate bivi.