I think a short, concise email will be fine. I also think a slightly longer, more personalized email will be fine, too, especially if you've been having conversations with these professors over the course of the app season. Just don't go overboard, obviously. Anyway, when I have to send out my own "rejection" emails, I'll probably opt for the more personalized approach, because I have been seriously hounding these poor POIs with questions and they've donated a lot of their time and very helpful/honest opinions to me and my crazy indecisiveness. This year, after writing a personalized withdraw email to a master's program, the prof responded with congratulations and tips on which PhD program I should choose for my interests!