hmm i never thought about tamron. i thought that they are usually more expensive than sigma hence i was only looking for the latter ones.
but after reading your reply i saw the results of tamron 28-75 on www.pixel-peeper.com
by the looks of it im assuming that tamron 28-75 is more closer to the results that of sigma 24-70.
i just checked, the price isnt that expensive for tamron. in uk its about £350-ish which is the same as sigma 17-70 but tamron 28-75 results look much more pleasing.
are there any drawback of tamron 28-75 how is it the same price as sigma 17-70. does it have any functionality defects or? can you comment on that.