Basically it is the same. There is no big difference between 양복 and 정장, but for me, 양복 sounds like old-fashioned compare to 정장. Maybe it's because 양복 is an old word made in open-door period, which means 'western clothes' literally. Oh, and there is a difference in that 정장 is word with no gender, while 양복 usually mean men's suits. In your case, i will use 정장 rather than 양복.

- 양복 = a formal suit. It is literally 양(ocean, overseas, foreign) + 복(clothes, attire), coined when the western suits were first introduced to Korea about 150 years ago. It is used more widely for men's suit. - 정장 = proper/formal attire (정 = proper, 장 = attire). It is a more abstract term, but usually means 양복. Women tend to prefer 정장 to 양복. While 양복 means the articles of clothing, 정장 can be both the physical clothes and the form of attire. So we say 정장하다 (dress formally), 정장을 하고 왔다 (came in a suit and tie), etc. where we can't directly substitute 양복 for 정장 but rather say 양복을 입고 or 양복 차림으로. 양복 seems to be used less nowadays except informally because it is not "foreign" anymore as suggested by "양". 산사복(men's suit) and 숙녀복(women's suit) and other terms are gradually replacing it. e.g. - (결혼식에) 양복 차림으로 나와 / 양복 입고 와 = Come wearing a suit (to the wedding) - works better for men. - 정장하고 나와 / 정장 차림으로 와 = (same) - works equally well for men and women.