一方 details

Examples

meanwhile; on the other hand — the second of two situations set side by side

都心としんの家賃やちんは上あがり続つづけている。一方いっぽう、郊外こうがいでは空あき家やが増ふえている。

toshin no yachin wa agaritsuzukete iru. ippou, kougai de wa akiya ga fuete iru.

Rents in the city centre keep climbing. Meanwhile, empty houses are multiplying out in the suburbs. neutral

兄あには朝あさから晩ばんまで働はたらいています。一方いっぽう、弟おとうとは毎日まいにちゲームばかりしています。

ani wa asa kara ban made hataraite imasu. ippou, otouto wa mainichi geemu bakari shite imasu.

My older brother works from morning to night. My younger brother, on the other hand, does nothing but play games all day. neutral

本年度ほんねんどの輸出ゆしゅつは前年ぜんねんを上回うわまわりました。一方いっぽう、輸入ゆにゅうは三年さんねん連続れんぞくで減少げんしょうしております。

honnendo no yushutsu wa zennen o uwamawarimashita. ippou, yunyuu wa sannen renzoku de genshou shite orimasu.

Exports this fiscal year exceeded last year's. Imports, on the other hand, have declined for three years running. formal

一方で
The clause-final 〜一方で hangs the second side off the end of the first clause, so both stay inside one sentence (雪ゆきが多おおい一方いっぽうで…); sentence-initial 一方 needs the first side already closed with a full stop, which lets the contrast reach across a paragraph break — either takes two different subjects freely, so the boundary is what decides which you write.
反面
反面 attaches to the end of a clause, and both halves have to be about the same TOPIC with opposite value — the good and the bad of one thing (便利べんりな反面はんめん、高たかい); 一方 stands alone before its clause and sets two separate things side by side, taking no view on which is the better.
それに対して
それに対して points back at the previous sentence explicitly and wants the two halves measured on the same dimension (百人ひゃくにんと三百人さんびゃくにん, 無料むりょうと有料ゆうりょう); 一方 only turns the reader's attention elsewhere, so it carries a loose pairing — two regions, two departments — that それに対して would make sound like a direct comparison.

一方だ
Same two kanji, opposite jobs — 一方だ sits AFTER a dictionary-form verb and tracks one trend that will not turn back (増ふえる一方いっぽうだ), while sentence-initial 一方 has nothing in front of it at all and simply turns to the other side; reading 一方 as "keeps on" at the head of a sentence loses the contrast entirely.