The thing to note is working distance.
Long lens can give good isolation through shallow DOF, but you need to get a feel of the working distance needed to take half/full body shots with such a lens.
The 50mm is nice as it does give shallow DOF for up to head to chest type portraits. (bkgnd needs to be far enough of course)
It also gives a very communicative working distance.
A 70-200 at say 135 and above on APS-C, you'd need to stand quite far away for half body and more.
Imagine trying to shoot like that in Orchard Rd with the people cutting into the gap between you and the subject all too often.
In the end, imo, probably need to get both to fulfill difference requirements/needs as they arise.