It really comes down to your comfort level with a tent provides the most space and coverage from the elements for you and your gear. However you still need to carry a sleeping bag so it overall it is the heaviest option. If you are sharing a tent with another hiker the weight is split with the other person carrying the cooking stove or whatever.
With a bivy shelter or tarp you also require a sleeping bag. Some bivy sacks are really a sleeping bag with enlarged covering over your head, although some styles are almost tiny tents and they are weather proof. You will not have space to change or clothes although some styles do provide enough space for you to read a book.
Hammock tents do protect you from the rain and bugs but it can take some getting used to sleeping this way although many hikers love them.
Another consideration is the time of year you are outdoors as in the summer months are three options work well.
Overall your decision will be based on the how much space you want or feel you need and the total weight of your lightweight outdoor shelter.
A bivy (bivouac) is slang for a small tent or shelter.
This is personal preference as both are good options. Many find a good hammock very comfortable while most prefer traditional tents or a bivy sack with a raised hood.
The best features of a bivy tent are that they are lightweight and compact so easily fit into your backpack or pannier. However the trade off is that they are not spacious so do not be able to stand and in some you will not even be able to sit up. Depending on the model there may or may not be space for your gear. But remember your shoulders will thank you for the lighter load.