The "f/1.8" vs "f/4.5-5.6" part is key as it denotes how wide open the aperture will go on each lens. The 50mm lens, with a maximum aperture of f/1.8, will allow you to shoot in much lower light conditions or with much faster shutter speeds than the zoom lens, which is designated "f/4.5-5.6" to let you know that the maximum (widest) aperture is f/4.5 at the 55mm end and f/5.6 at the 200mm end.

To compare the 55mm end of the zoom lens with the 50mm lens, if you took a photograph at 1/125s at f/1.8 with the 50mm lens you'd need to use a shutter speed of 1/15s or slower with the zoom lens in identical lighting conditions (or change the camera's ISO setting by three stops to compensate), as it will not open wider than f/4.5. This makes the lens much less versatile as far as lighting is concerned. Additionally, using the f/1.8 aperture on the 50mm lens will result in a much shallower depth of field, something that is often highly desirable in portrait photography.

The 200mm zoom range would not be much use for formal portraits either, as your subject would usually be close enough to allow you to shoot at 55mm.

My advice would be to buy the 50mm lens if its primary use is for portraiture. The zoom lens sacrifices a lot for that zoom range and should only be used if you need the full range of the zoom and you have a restricted budget.