dylee8 wrote:
....Also the D750 is 24 megapixels, compared to 16 on the D7000. Even considering the sensor size differences, the "Megapixel Density" are identical. This means that if I start cropping the pictures from both cameras, I should get the exact same resolution - even though I am cropping more on the D750....
This is NOT correct.
In DX mode, the D750 is reduced to a 10MP camera (compared to the D7000's 16MP or the D750's FX mode 24MP... If, instead, we were comparing with the 36MP FX D810, it's DX mode image size is a bit more than 15MP, so would be more comparable to your D7000's resolution.).
So there is significant loss of resolution, using a full frame camera in the "crop" mode. More than you might think, at first glance. And you'd see exactly the same degree of IQ and resolution losses if you were to crop the image to the same dimensions in post-processing.
Because of this, using the 150-600mm on the D7000 makes more sense for wildlife photography. It's 16MP gives you potential for more enlargement or more cropping, if needed.
That said, 10MP may be "enough" for a lot of purposes. Heck, I got some good shots with 1.2MP cameras in 1998, 6MP cameras in 2004 and 8MP cameras in 2007!
But keep in mind, too, that the D7000 is now a five year/two generation old camera model. It was introduced in 2010, superseded by the D7100 in 2013.... and that camera was in turn superseded by the 24MP D7200 last year (though this was a less dramatic upgrade than the D7100 vs the D7000).
Part of the D7000's greater resolution (compared to a cropped D750 image) is likely offset by the continuing improvements in DSLR tech... four and five year newer models most likely render a bit better overall quality, in addition to any sheer increase in resolution.
In comparison, the 24MP D750 was introduced in 2014 and is still in production.