70-200F2.8 or Tamron 70-300F4

[Dangermouse]

Last week at Lakenheath I noticed most people were using 70-200F2.8 instead of 300mm lenses, I was using my Tamron 70-300F4 VC and getting some pretty decent results, would it be better using a 70-200 Canon lens and cropping or carry on using the Tamron as some images from the 70-200 lenses are pretty amazing and IQ is much better, or seems it.

 

[snerkler]

Last week at Lakenheath I noticed most people were using 70-200F2.8 instead of 300mm lenses, I was using my Tamron 70-300F4 VC and getting some pretty decent results, would it be better using a 70-200 Canon lens and cropping or carry on using the Tamron as some images from the 70-200 lenses are pretty amazing and IQ is much better, or seems it.

I assume you mean Tamron 70-300 f4-5.6 as I'm not aware of an f4? If so then the difference in IQ between the 70-300 and 70-200mm f2.8 quite a lot, but then so is the cost and the weight. You're comparing a budget lens (although good one) with a pro level lens. Yes you will lose some IQ difference cropping 200mm to match 300mm, but from my experience (albeit comparing the Nikon 70-300 f4-5.6 and 70-200 f2.8) the 70-200mm still gives better results. They're not 5-6 times the cost better though IMO.