I've done some very informal portraits with the DA 50-200, and it's more than adequate in that application. My copy starts to lag when I zoom out past 150mm or so, and I always try to stop down to f8 - f11. I'll be doing a more controlled portrait shoot this Sunday (mostly as a test of the $1.80 Gary Fong clone that I made for my 540FGZ), and I was planning on using the DA 50-200 for most of it, though I'll also try my SMC-A 50mm f1.7, both with and without a Tamron 1.4x TC.
In your original post you state that you don't need 300mm, but do need 50mm for portraits, so it looks like you've narrowed your choices already. I have no experience with the Sigma 55-200, so if you have the chance to try it with the DA 50-200 side by side, take it, and let your eyes be the judge.
All that being said, if I could justify (to myself) owning the the DA*50-135, I'd be all over that like me on a pint of Guinness!