The real questions should really be what sort of portraits do you want to take, full body, half length, head and shoulders? How much room do you have to shoot in? Really you need to be at least 10 to 12 feet from the subject, to avoid perspective distortion. I would much rather shoot at about 15 feet if I could. You then need to choose a focal length that will allow you to frame the subject in the manner that you want. This is especially true in the studio, where the available distance can be quite restrictive. In that case it may be that you will have to compromise and use a shorter FL so that you can frame the longer shots. Then if you need to do a tight head and shoulders shot you will need to crop the image to get the desired framing. This is a situation where you want to avoid getting closer to fill the frame, as the closer shot would introduce perspective distortion. Remember that it is ONLY the relative placement of the various parts of the image, in relation to the camera, that cause the distortion. NOT the focal length of the lens.
If you are shooting outdoors in a situation where you have room to back up some, then using a longer lens is not such an issue. You have the option for getting a tight head shot from a normal distance, and still have the opportunity to back up to a much greater distance for the full length shot. The increased distance involved gives a much flatter looking image, with considerably more background blur thanks to the changes in perspective.
So only once you know what you will be shooting, and from where can you really decide what lens you need to use. If you try to pick the lens first it will be dictating to you how you want to take your images.