Most of my bikes have level top tubes, but two of them have sloping ones. The tandem has a slight slope since it was built with a slightly shorter seat tube in the back - we actually don't need it that way, but it's a very common tandem design since most couples aren't the same height. And my folder has a sloping top tube (or no top tube at all depending on what you call the single tube joining the head and seat tubes). On one tour I did with the folder my friend commented that he was envious of how easily I was able to get on and off - a fully loaded touring bike makes it a little harder to swing a leg up and over the seat.

But I don't see much of a functional difference either way. If you prefer the traditional aesthetics of a level top tube then I'd suggest you stick with that.