I had a 55-200 and moved over to the Sigma 70-300 APO. Don't bother with the non-apo...get the APO version.
My thoughts. The 55-200 is sharper throughout it's range...however, the Sigma is very good in the 70-200 range as well getting softer the closer you get to 300. Keep the sigma stopped down to f/8-f/12 in good light and it's still a really sharp lens.
Sigma 70-300 APO - 170mm f/8
Sigma @ 300mm f/8 w/ some post sharpening
However, the 55-200 is excellent too and I'd give the edge in IQ to theh Nikon
Nikon @ 200mm and f5.6
Where the Sigma REALLY shines is the macro feature. It is very sharp in macro mode and stopped down with a good flash can get you some really good shots. Plus 1:2 and 300mm is great for bugs.
Sigma - 195mm and f/25....VERY sharp
Sigma - macro mode and 300mm f/16
So you need to decide. Is the macro feature important to you at all? If not, I'd go with the 55-200, if the macro is something you'd use, I'd recommend the Sigma....this thing is a crazy sleeper in the macro area.