I prefer a level top tube - one of the reasons that I bought my Surly Long Haul Trucker.

In my opinion the sloping top tube allows too much of the seatpost to be unsupported. When you climbing a steep hill, and are down in a lower gear and really "stomping" your way up, I think that the extra exposed seat tube is just acting as a lever with a longer moment arm, trying to pry your clamp and seat tube apart. But, keep in mind that I'm a big guy - about 255 lbs - so I am concerned about such things. (I picture myself pedaling hard up a hill....seat post breaks......Tom visits Proctologist to have seat tube removed from nether regions!)