Your lenses already cover the range from 18 to 200 mm.
Therefore, and this is my opinion, I would not buy a lens that covers almost the identical range.
The only thing you would gain is that you wouldn't have to swap lenses.
All-around lenses always have drawbacks, replacing your two lens setup by one superzoom, would probably only hurt image quality.
Concentrate on your style of photography and see where the real limitations are for you. Those could be: limited range, low-light shots, macro, etc.
I hope that somone else can comment in here too.
Side note:
The Tamron 18-250 is about 500 us dollars i think.