im actually considering the tamron as the betetr option. just been looking on TDP (The Digital Picture ) and a side by side comparison of the tamron 150-600 over the canon 100-400 and sigma 150-500.
set the scale at 400mm f/8 for my camera and the tamron is significantly sharper than the other two.
even dropping down to 300 mm on the crop sample the tamron is much much clearer
http://www.the-digital-picture.com/...meraComp=453&SampleComp=0&FLIComp=2&APIComp=2
kind of surprises me but then i guess the canon 100-400 ( not the version 2 ) is a few years old now.
ive also seen a used Tamron for sale purchased only a few months ago for £550 , as the only place i can find it new in budget is places like simply electronics... and im very distrusting of hongkomg/ china based companies ( plus the added risk surprise of being lumped with hefty import duty and tax is not a risk im willing to take.
cant find it anywhere in UK even close to £600 ( i may be able to stretch to 650 at a push ).
really seems to be 4 options.
canon 100-400
sigma 150-500
Tamron 150-600
or a shorter lens with a 1.4x tele ( if i can find one in budget ( a 300 lens will give me 420 but much prefer the reach of a 400 with a 1.4 which will give me 560 but then a decent TC could cost half as much as the lens!