Ray,
I used Sigma 150-500 on D300, and was happy with the results. The stabilizator is not as good as on Canon, and at the 500mm the lens gets softer, but at 400 and 1/1000s or faster it's pretty good.
Theoretically, if you mount this lens on D3200 and a supersteady tripod, at 400mm and F6.3, you might see some fine details better resolved than with D7000.
However, in practical terms (shooting handheld and at 500mm), I wouldn't expect big differences between those two bodies attached to Sigma 150-500.
Les,
I don't think I've ever seen so much confusion on the internet, regarding a comparison between two lenses. You say the stabiliser is not as good as on the Canon 100-400, yet others say it is marginally better because the Sigma is a more recent model of lens with perhaps as much as an extra stop of IS.
Others complain about the extra weight. The lens alone is certainly heavier than the Canon 100-400, but here again there is confusion because the weight of the Canom 100-400 seems to have been reduced in more recent production copies. My copy of this lens definitely weighs about 1.6 to 1.65Kgs. Yet the current specs for this lens, on the USA website, and others, mention a weight of 1.38Kgs.
The D3200 body is lighter than the Canon 50D body (or D7000 body), so total weight of the 150-500/D3200 combination would be about the same for me, or even marginally lighter.
The main point, and most disturbing point, is that the 150-500 is softer at 500mm than the 100-400 is at 400mm, at full aperture. But here I believe there's also confusion.
My copy of the Canon 100-400 was bought in 2002. At 400mm it's definitely softer at F5.6 than at F8 ot F11, as indeed the Photozone tests show with the 20D. In fact I used to think my lens was sharpest at F11, until I did some controlled tests and found it was very marginally sharper at F8. The impression I'd initially got that it was sharpest at F11 was probably due to the lesser need for critical focussing due to the greater DoF at F11.
However, the sharpness difference between F8 and F11 is so slight, the main reason I use F8 with the 100-400 at 400mm is because it allows a faster shutter speed at the same ISO.
I've read reports that recent copies of the Canon 100-400 have improved optics and are now at least as sharp at F5.6 as at F8, at the long end. If this is the case, I can understand why the Canon zoom is considered to be sharper at 400mm, comparing both lenses at full aperture.
However, it seems to me if one is serious about resolution, one should always try to find out at what aperture one's lenses are sharpest. The Sigma 150-500 at full extension seems to be sharpest at F11, so that's the F stop one should always try to use if one wants the sharpest result at 500mm.
When a lens is described as being sharpest at a particular aperture, one needs to know 'how much sharper'. Whilst a Canon 100-400 might be noticeably sharper at F5.6 than the Sigma at F6.3, it might not be noticeably sharper than the Sigma at F11. Just as a lens can be very marginally sharper at F8 than it is at F11, it can also be only 'very marginally sharper' at F5.6 than at F8, or even equally sharp at F5.6 as it is at F8 and F11.
It's a pity that Photozone hasn't tested the Sigma 150-500 yet. However here's a test using the D800 with the lens, showing various test charts indicating that the lens is sharpest at F11 at 500mm. One might be able to get equally sharp results F9 or F10, but F6.3 is clearly much softer.
http://www.cameralabs.com/reviews/Sigma_150-500mm_f5-6-3_DG_OS_HSM/
