Start with a body wash and then a body scrub to remove any dirt, grime and sweat from the surface of the skin, and then remove dead skin cells and impurities. In addition, many body scrubs are also very moisturizing, and if you use body gel first and then a body scrub, the advantage is that you'll leave the moisturizing ingredients of the scrub on your skin, rather than washing it off.
On the other hand, there are benefits to applying a body scrub before shower gel. If you're an oily skin type, the moisturizing effect left behind by most body scrubs can leave your skin feeling too heavy and greasy, which is why it's best to shower afterward. If you start with a scrub and then wash, you will definitely remove any residue in the scrub, including fine particles. The benefits of using a body scrub before showering include cleaning freshly exfoliated surfaces, which also means that after exfoliating, you can wash away all of these impurities and dead skin cells.
It's ok to use either a body wash or a body scrub first, but depending on what they're used for, the body wash can be used daily, but body scrub can be used once or twice a week.