"これはあの人の時計です" This is the answer to the question: "Whose watch is this?" "これはあの人の時計です" = This is HIS/HER watch. In this situation, it's focused that to WHOM the watch belongs. "あの人の時計はこれです" This is the answer to the question. : "Which (one) is his/her watch?" "あの人の時計はこれです" = His/Her watch is THIS. In this situation, it's focused that WHICH ONE belongs to the person. ---- Hope this will help.

There are some watches on the table. You point to one watch. You can sayあの人の時計はこれです. There is a watch on the table. You can sayこれはあの人の時計です. It’s depends on the situation, so we use both. Sorry, my English is poor. I hope you would understand.

If I'm not mistaken, these sentences differ in what is the new information they give. これはあの人の時 計です。 - In this sentence "あの人の時 計" is a new information about "これ" - the topic of the sentence. It could be an answer for "これは何ですか。" or "これはだれの時計ですか。" あの人の時 計はこれです。 - The new informaition is "これ", and "あの人の時 計" is the topic of the sentence. It could be answer for "あの人の時 計はどれですか。" Hope it will be helpful.