A couple of decades ago, choosing between a one-person tent and a bivy sack was pretty easy. The differences between the two types of solo shelters were more extreme than they are now. Early bivy sacks were little more than oversized trash sacks: ultralight, simple and cheap. However, they weren't breathable, and unless it was pouring down or you didn't mind waking up wringing wet with condensation, you were probably better off not using it. Tents, even the lightest models, were comfy, but impossibly heavy.
Fast forward to today and the choice is much tougher. Modern bivy sacks, thanks to the development of breathable fabrics, are now pleasant to sleep in. Meanwhile, some solo tents are ultralight as well as strong and durable. It means there's a lot more to weigh up before you take on your adventure.
