Sammy is definitely a nickname to me. The way I think of it, is when he's thirty, how is he going to want to introduce himself? At least if you put his name as Sam, not Samuel, it's more grown up. My BF has a friend whose name is Andy. Not Andrew, Andy. And he has to introduce himself like that. He hates it. Just my opinion. Hope it helps and you find a name that you really like!!