A bivy bag and a tent solve different problems. A tent gives you room to change, organise equipment and wait out poor weather. A bivy keeps the sleeping system compact and close to the ground, with less fabric and fewer parts to carry.
Choose a bivy when a small packed size, low profile and quick setup matter more than living space. The Carinthia Combat Bivy is a straightforward outer layer for a sleeping bag. The Observer Plus adds a more structured head area, while the XP II Plus offers another compact shelter format.
Condensation deserves attention. Moisture from your body has to move through the sleep system, and ventilation still matters in breathable fabrics. Avoid sealing every opening unless conditions require it. Air and dry the bivy and bag whenever you can.
A tent remains the practical choice when you need protected storage, expect long periods of rain, share shelter or will spend more than sleeping hours inside. A bivy can also work under a tarp, giving you rain cover and a small area to manage gear without carrying a full tent.
Check the packed dimensions, entry arrangement and compatibility with your sleeping bag before buying. The best option is the one that suits the whole trip, not simply the lightest item on the scale.