反面・一方で・に対して: the difference
While, On the other hand, Although, However, On the flip side
Tested 2 times in past papers
都内に引っ越して色々と便利になった反面、家賃が前住んでいた所の倍になった。
While moving to Tokyo has made many things more convenient, the rent has doubled from where I used to live.
On the other hand, At the same time, While
Tested 5 times in past papers
この靴は、温かい一方で、重いです。
These shoes are warm but on the other hand they are heavy.
In contrast to, While, In regard to, Whereas
Tested 6 times in past papers
彼はみんなに対して優しい人なんです。
He is kind towards everyone.
How to tell them apart
「反面」 is only for two sides of the same thing; you can't compare two different people: not 「×兄は優しい反面、弟は意地悪だ」.
「一方で」 is wider: it can also mean one person doing two things at once, or two parties each doing their own thing.
「に対して」 contrasts two different things: 「都市部が過密なのに対して、地方は過疎化が進む」 (Cities are overcrowded, while rural areas are losing people).