The 50/1.4 is considered to be a sharper more modern lens. It is a Double Gauss design and shares the those qualities of a number of modern lenses. It has more modern coatings, and is much bigger than the 50/1.5.
The 50/1.5 is a true Sonnar copy and has all the qualities of a Sonnar. The images are sharp in the center and very smooth and pleasant out to the edges. It produces a much different classic image. It is TINY. As small as the collapsible Summicron, collapsed. The 50/1.5 has a build quality as high as the best Leica lenses. And I think the chrome barrel is cool.
Is the 50/1.4 better? Maybe. My personal opinion is no, but if I am right that is just saying a whole lot about how much I like the 50/1.5 because the 1.4 is very very good.