If you're choosing one over the other, I think you should choose the conditioner. It helps your hair, while as shampoo just cleans and dries it out. Honestly, I hardly EVER use shampoo. Maybe once every 3-5 months, call me gross but I like my hair healthy. From what I recall, I think you posted something about your hair being super thin ? I advise you use some conditioner.
It moisturizes your hair, adds back the oil into your hair after it was stripped off from the shampoo. It also helps detangle, smoothen and make your hair shinier and sometimes helps with frizziness.
It makes your hair easier to brush, and play with.