^ What he said.... if you use 600 on an FF and 400 on APS-c, you have exactly the same ability to crop on both systems as long as they are 24 Mp. The 6000x4000 pixels for the D750, 6000x400 pixels for the K-3, The number of pixels is the same on both....so if lenses are equivalent, you need a much more expensive lens on FF to get the same field of view. IN low light you may have a bit of an advantage with a D750. If DoF is is important, then you are going to get more DoF using APS-c for a given ƒ-stop.
Hey, if you want to go for a full frame, don't let me talk you out of it. But I have the reach you plan to achieve with a D750 and 150-600 with my $500 SMC A-400. There may be other features that make the D750 attractive. I don't know I've never met a D750 shooter up here in wildlife land. But it's a new camera so, maybe soon.
For me it's really simple, there are no 600mm lenses that I'm capable of carrying on hikes, because of weight, and when you go to a bigger lens, you need heavier tripod. I don't know about the 150-600 it seems like a goo lens. But I seriously doubt it's DA* 300 + TC quality, or DA 560 quality. I'm sure if I'm wrong someone will correct me.
Maybe the 150-600 is better, and it could be marginally better on an FF. But my experience with Tamron and Sigma long zooms is that they are pretty bad in the long end. For me, send them back to the store and get my money back bad, but, who knows maybe Tarmon hit a home run with this lens and things will work out for you.
Check out the goose, further down this page,
https://www.pentaxforums.com/forums/122-lens-clubs/55946-300mm-plus-lens-club...ml#post3062629
Last edited by normhead; 12-07-2014 at 08:22 AM.