I have both the 50mm 1.8 and 1.4. If you are serious about photography, and if you are making money doing it, I would without a doubt go with the 1.4
But back to your original question. Are you looking for something to replace the kit lens, or to compliment it? The tamron 17-50 is a great choice to replace a kit lens with pro quality glass, however a nice prime is great for portrait.
The 50 and 85mm lengths are both popular for portraits, the 50 is better suited to wider shots, and the 85 is better for headshots, not just because of focal length, but because of the perspective distortion that is inherent with the focal length. Longer lenses compress features and this is more appealing in head shots and tighter portraits. Many photographers use even longer lenses in the 70-200 range.
If your kit lens is still doing the job for all around shooting, than I would look into the 50mm 1.4 to add to your collection, USM focusing, solid build, smooth bokeh, and super sharp, basically its everything that a kit lens is not.
The 85mm 1.8 shares all of these qualities, but is usually not suited for indoors, and the working distance can be prohibitive in many non-head shot portraits. But if headshots are your thing than this is your lens.