They need to make an affordable 150-500/4-5.6 or better, DX. I doubt they ever will, but if they did, I'd expect it to be much smaller, lighter, and much more affordable because of that. Een though this is already pretty affordable when you realize what you're getting out of it. sigma wins hands down out of these two. The Sigma 50-500 is even better in terms of IQ.
I completely agree with you Mark regarding 150-500/4-5.6, they should come out with one. 80-400VR has long passed it's life cycle and I have heard that it is going to be upgraded. My two cents on this that there will only be a AF-S tag with this lens and obviously few $$$ higher pride tag. Still it will not be compatible with even a 1.4x TC making it's zoom range unattractive to bird photographers.
Regarding Sigma there seems to be copy to copy variation. I agree that 50-500 OS seems to be doing better but the problem is that it costs quite a bit high, about $600 and I already have part of it's zoom range in my bag and that also from Nikkor, so actually it will be wastage in a way.
Between Tamron and Sigma 150-500, the
main differences that I could spot are as follows:
-----------------------------
Tamron 200-500 ------------------
Sigma 150-500 OS
Max Aperture (wide): ----------f5 -------------------------------------- f5.6
Min Focus: ------------------2.5m ---------------------------------220cm ------- Interesting to note,
Weight: ---------------------1237g -----------------------------------1910g ------- Interesting to note
Stabilization: --------------------X -----------------------------------------Y
AF Motor: ----------------------- X ----------------------------------------HSM
Min Aperture: ----------------32 -----------------------------------------22
Lens Elements: -------------13 ----------------------------------------- 21---------- No idea which one is optically superior,
is it lesser the better?
Lens Groups: ---------------10 ------------------------------------------15---------- No idea which one is optically superior,
is it lesser the better?
For bird photography, it is more common to use Sv >=1/500 and at this shutter speed OS really does not matter, moreover at high shutter speeds it is recommended to turn the OS off. I have also seen that for BIF and other type of specific bird related scenarios, OS simply slows the focus down, unless it's a bank breaking lens like Nikkor 500 f4 VR. So for these low budget lenses, how much OS is helpful, I am not really sure!
Now,
which one looks optically more impressive?