Sales Cloud is designed to help your sales team sell smarter and faster. It allows them to manage your company's contacts, accounts, leads, opportunities, quotes, and deals easily, all on one platform. On the other hand, Service Cloud helps your company run and improve customer service using accounts, contacts, cases, and solutions. It's designed to help support your past, current, and future client service requests to help you retain your existing customers. Which app you need depends on which area of business you are trying to improve.