Ian, if you want the flexibility then the 50-140 is a good shout, added to the fact that the TCs will work with it, you can get your telephoto range back if required.
I don't do many portraits, and I had the 56, (at the time I had 14/23/35/56/90 and a bunch of other stuff), I much prefer the 90, for me its quite a special lens with beautiful rendering and very snappy AF (the 56 AF was always a little pedestrian in comparison). I love the 90 and find it a surprisingly versatile lens (and that's coming from someone who is predominately a wide shooter, I'm actually now down to 12/16/23,18-55/90 and happy with my selection)
Disadvantages of 50-140 are weight (at nearly 1kg its no lightweight) and the min focusing distance of 1m (the 90 is 0.6m and the 56 0.7m)
You'll welcome to pop over and try out the 90mm if you want, just PM me