Eliot said they are the same, and from a grammatical point of view he is correct. Mitong's answer is excellent and provides an excellent grammatical explanation. However, a native English speaker will notice a small (subtle) difference between the two, and Canadians who are known to be respectful and perhaps even too polite, would notice a slightly bigger difference between the two expressions. "Please sit down" -- the word 'please' gets the listener's attention in a polite way, shows respect to the listener, and 'sit down' at the end then sounds like a request or an offer to make yourself comfortable. "Sit down, please" -- 'sit down' sounds like an order or a command comes first, and then 'please' at the end sounds like an afterthought (something you add at the end to make the command or order less forceful (i.e. less demanding). So, at least in this Canadian's opinion, "Please sit down" sounds more respectful, gentle and polite.

In an English imperative sentence, the polite word "please" can be put: 1) at the beginning of the sentence, or 2) at the end of the sentence. If it is placed at the end of the command, you may also add a comma before please for emphasis, e,g., Sit down, please. In a question format, "please" is added between the modal verb and the infinitive. For instance, "Can you please give me that book?" Good weekend. / 良い 週末を。

The same.

both are same but ' please sit down ' sounds better,