I am weighing up my options as I feel I want a mid zoom with a bit of speed, mainly for portrait/candid/wedding..without having to switch as much. Currently have a 50/1.8, 10-20 and a 70-300 on my D300, (in the future I am looking to go for a F2.8 in the 70-200 range) and ideally would like something so I am flexible when shooting relatively close to my subject and as a bit of a walk around (though I do love my nifty). I really like shooting with the 50...but sometimes the situation doesnt allow you to take that few steps forward or back...and sometimes means you miss getting the moment you are after.
Hence I am considering the Sigma 24-70, the Tamron 28-75 or the Nikon 24-70 F2.8. Having read a number of reviews I am still not sure and was hoping that people might be able to offer their personal opinion and experience on any of these lenses and whether they have compared them. Ideally I will be trying each of them but it would make it easier if I had some sort of idea about what to look for in that testing process.