The best would be 70-300L + 135L, but I understand you do not want to sell the Sigma 70-200. You should then ask yourself what it is you want to achieve with the new lens that the 70-200 cannot do. Do you want more reach, get the 70-300L. Do you want to increase shutter speed in low light, or improve somewhat bokeh at 135mm, get the 135L. Note that the 135 at f/2 provide background separation similar to that of the 70-200 at 200mm and f/2.8.
135mm f2 - for stunning portraits (I have a 85mm 1.8)






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- Bad news - the weathers rubbish 

