If you can play one kind of sax, you can pretty much play any of them. The fingerings are the same, and tone production is basically the same. It's just a matter of getting used to the difference in size, etc.
EDIT: Oh sorry, I'd forgotten what the question was. As far as ease of mastery...There may be an advantage to learning alto first before soprano. I really don't know. One practical matter--you may want to start out playing on a student model, since they're less expensive, and they can also be rented. And I wouldn't think they make student model sopranos.