Canon's EF 85mm f1.8 would be my choice of affordable portrait lens, it's of a good focal length and is fast to throw the bg out of focus. Good and sharp, light and quick to focus.
I have the 50 1.8 but generally use my 135mm prime for portraiture over is as the 135 allows you to get further away, and has less pincushion distortion, so you don't end up with all your portraits looking like those pictures of dogs noses taken with a 14mm fisheye!
An f/4 lens really isn't that fast, and so the DoF at 70mm will still be quite deep, the 70-200 is also considerably larger and heavier than the 85mm prime which may be a factor in your decision.
Col