Sag. When a valve amp is pushed hard at high volume, it draws lots of current, which the power circuit can't deliver quickly enough. This creates a small amount of compression at the start of the note, which then blossoms into the full note in a very pleasing manner.
For me, it's not the case that one is better than the other, more that each has strengths and weaknesses. I have one clean SS rig (well, I will when the cab arrives), and one all-valve rig.