It would be much appreciated if you could let us know your preference at your earliest convenience.
It would be much appreciated if you could let us know which plan you would prefer at your earliest convenience.
相手が大事な顧客あるいはクライエントと想定して表現してみました。メールであればもっとカジュアルでもいいとは思いますが、
It would be much appreciated if you could let us know your preference at your earliest convenience.
貴殿(貴社)のご都合の良いときになるべく早くご希望をお聞かせ願えれば幸いです。
It would be much appreciated if you could let us know which plan you would prefer at your earliest convenience.
貴殿(貴社)のご都合の良いときになるべく早くどのプランをお好みになるかをお聞かせ願えれば幸いです。