I own both lenses, and have for several years. As far as a portrait lens goes, the Canon is the better lens--it focuses better and more accurately and repeatably on portraits or action subjects. The Tamron will not autofocus indoors in low light nearly as well as the faster aperture Canon will. The Tamron is a macro lens, and it has a long, slow focus throw in both AF and MF, for accurate and precise close-range focusing. At typical portrait distances, the Tamron suffers from the hair-trigger Infinity-to-Three-Feet-in-10-Degrees syndrome that most macro lenses suffer from.
Most macro lenses have extremely short focusing throws from Infinity and back toward about 1 meter; this can lead to erratic, inaccurate focusing, both in AF or in Manual focusing, slow focusing, and a tendency to hunt for focus a LOT on fast-moving subjects. A tele macro like the Tamron 90 is not a good choice for action shooting, or even fast-moving kids...it's designed to focus slowly but precisely at CLOSE ranges. The 85/1.8 is a portrait and field telephoto lens, capable of shooting moving subjects in any light level. The two are very different designs and are optimized for entirely different work.
For example, at an indoor sports event, or any sporting event, using Continuous AF: the macro lens will have an AF hit rate probably 30% below that of the Canon 85mm lens. Same for studio shooting under modeling lights--the macro lens is simply NOT optimal at beyond 3 feet in terms of focusing reliability--it simply does not work as well as a field telephoto.
