I have both these lenses. Actually,I did what your suggesting. I sold my 50-200 and bought the Tamron 70-300. However, I relented and purchased another 50-200.
I find I have use for both and am much happier in choice owning both.
Have seen many questionable reports about the Tamron, build, PF, softness etc.
I dont agree with the questioning of the build. I feel it is quite a well built lens.
The macro I find good. Purple Fringing, yes, in bright contrasty light, but closing down(F8 and smaller) tends to overcome this. Softness at longer end? Hasnt caused me a great deal of concern up to date.
Then again, maybe I am a bit easier pleased than some, maybe I havent raised my standard bar high enough!
I canr ecommend this lens, considering its price I feel its a worthwhile purchase.