(I am teasing you:) You seem to be seeking a hard rule that you can apply without thinking, but there are no hard rules, certainly no One rule, and thinking is always helpful. :) It always depends, on the situation, and on what result you want.
70-90 mm is "ideal", if assuming a cropped APS sensor, and assuming a normal subject distance of at least 6 to 8 feet (for proper portrait perspective), and it is not a group shot. But even better, the 70-300 will also do say, 150 mm or 200mm, which from say around 15 or 18 feet (2x focal is 2x distance) is still ideal, still with proper perspective.
And (speaking about outdoors) if focused at this 15 feet, it will blur the dickens out of any background that is reasonably distant. Plus, as important, it sees a much more narrow view of the background, so that moving slightly can easily choose a better one.
IMO, beginners go for the f/1.8 lens, because they are inexpensive, and they heard they should, (do you really like f/1.8 portraits?), and because they normally stand too close for perspective anyway.
IMO, pros probably more often seek the longer lenses from the longer distances, which are not f/1.8 (another plus), and they do the job better, good perspective, and better background choice.