I. 衣服, an informal term, we use it a lot when you go to buy clothes, or you express you need to wash your clothes. eg: 买衣服，洗衣服，穿衣服，换衣服 这件衣服脏了。 这件衣服多少钱？ II. 服装, is a formal term, when we talk about fashion magazine or garment, we use this term. eg: 服装展，服装设计，服装设计师，服装产业，服装设计费 他是一名著名的意大利服装设计师。

服装 is more formal. You don't use it in Spoken Chinese very often. Chinese usually use "衣服" in their daily lives. For example：服装产业 garment industry 我现在穿的是红色的衣服。I am wearing a red clothes now.

服装 is more official,and in day-to-day spoken language,it is seldom used. e.g.服装店，服装品牌 衣服 is often used in our daily life. e.g.你得穿好衣服，外面冷。 这件衣服真好看。

“衣服”可能不只是informal，我觉得用common比较好，而“服装”用“professional"更好一些。