What is the difference between a horizontal top tube and a sloped one?
I thought this was an interesting question, so first of all, +1.

First off, the sloping tube (your second image) is known in cycling parlance as a compact frame. I found an article on the Giant web site about the advantages of a compact frame. When I say "advantages" - this is Giant's word not mine!

The full article is here, but to summarise it:


the compact frame is smaller than the corresponding standard frame. Therefore, less material is used in the manufacture. Therefore, it is lighter.
Again because it is smaller, it is stiffer
With traditional designs, to stiffen the bike designers use stiffer materials which unfortunately makes the ride rougher. By using the natural stiffness of the frame, materials don't need to be stiffened hence the ride is comparatively smoother
because the top tube is lower, it is easier for cyclists to get a good fit


Now I'll say straight away that you can attach your own weight to some of these features, this article has a whiff of marketing about it. But it certainly gives you some food for thought.

Interesting also to note that Giant's current top-of-the-range mens' road bike (the Propel) has a standard frame (i.e. horizontal top tube). Take from that what you will!