They are similar, but (으)ㄴ/는 반면에 is more specific and technical about the contrasting points. '-지만' is like "although" or "but". It's a common contrasting or contradicting verb ending (e.g. 저 배우는 잘 생겼지만 연기는 잘 못해 = That actor is good looking but he can't act very well.) '반면에' in '-(으)ㄴ/는 반면에' literally means "opposing(반) point/aspect(면)". So it spells out the opposing points more formally, like "in contrast to (something)" (이 정책은 경기를 부양한 반면에 통화 가치를 떨어뜨리기도 했다).