Tamron 18-270 or Canon 70-200L f4? – Well, if you have the money, jrarsenault advises to buy both, for the following reasons:
I would recommend buying the Tamron in "addition" to the Canon 70-200, not replacing it. In photography, each lens has a purpose. For me, the Tamron is a travel lens because it's IQ is respectable and it is very convenient as a one lens solution. However at home, I would use the 70-200 because of its additional IQ (over and above the Tamron), and the brokeh it creates is down right beautiful (the Tamron cannot compete here).