What type of portrait situation are you looking at?
Street candid portraits? 1 to 1? Group model shoots? Free ones like Auto shows?
It will influence what is best used.
I give you the option that will settle all this and is quite cheap either brand you buy.
Sigma 70-200/2.8
Yes. No joke. Its good enough for many wedding photographers its good for you and me.
For a full body shot with good subject isolation the long end of the 70-200 (ie. 135-200mm) is the way to go.
I don't like full body shots too much, but here are a few that I can dig up from my limited photos that I have.
Not exactly fair comparisons since focus distance is different, bkgnd distance is not exactly the same.
135mm at f2.5 or f4
85/1.4
300/4.5
This one not human portrait, in fact he is smaller and the subject isolation is better, but hope you get the idea.