I have about £650-700 to spend on a lenes for my D90. I'm turn between sigma 50-200
f2.8 , 150-500mm or 120-400mm or open to surgestions?
OK...well very different lenses and the difference between a 200mm lens and 500mm lens is HUGE..
You need to tell us what you want to shoot.
Never buy a short lens, with the intention of using a TC. That is nature's way of saying you just need a longer lens
Put the Nikon on your list, that is an excellent lens.
Just be careful of those dodgy apertures. I find it annoying when your at f/4 at 75mm then zoom to 300 and youve been forced to 5.6...
If you are going to get a lens try to get it with a fixed aperture.