I don't know how you use "boutique" in the US, but it's a commonly used word around here (when speaking English anyway).
I do think there is definitely a qualitative difference in the way people speak in Europe and in the US (also when speaking languages other than English).
I was a bit surprised when I have heard a (supposedly, could have been Canadian AFAIK) American tourist saying something like "they were just a bunch of dudes hanging out" (she was a woman in her 40's, in Austria using an equivalent form of slang is something normally reserved to much younger people).
It was always amusing to me when my British drama teacher would use a word that Americans rarely use like "coach" or "metro" (as opposed to bus or subway). I had just big enough a vocabulary for it to be no effort for me to understand her, even if I never used such words personally, whereas everybody around me just went "huh?" I guess I'm finally noticing the times when I'm on the other side of that divide and have to really think about what the unfamiliar words mean in the given context.
I do think there is definitely a qualitative difference in the way people speak in Europe and in the US (also when speaking languages other than English).
I was a bit surprised when I have heard a (supposedly, could have been Canadian AFAIK) American tourist saying something like "they were just a bunch of dudes hanging out" (she was a woman in her 40's, in Austria using an equivalent form of slang is something normally reserved to much younger people).