First things first, consider the space you want to be sitting in. What does the space do? How big is the space? The difference between a sofa and armchair is function and size after all.
Space is a huge consideration when it comes to furniture, obviously a sofa is bigger than an armchair - so for a smaller space, go for an armchair. If you’re looking for a piece for your living room, but you haven’t got the space, a two seater sofa is perfect.