Document preview: iPad wins
In the iPad's iOS, you access documents from within a compatible app, so there's rarely the issue of a document you can't open natively. But when you get an email with an attachment, you're not in any such app. iOS's Quick Look facility comes into play, opening the document so that you can go through its contents. (Supported formats include PDF, Office documents, iWork documents, RTF files, text-only files, and most popular graphics files.)
A nice aspect of iOS's approach is that you can open files in supported apps or in Quick Look, as you prefer -- and you can open a document viewed in Quick Look in a compatible app, should you want to do more than view it -- just as in OS X.