Here are a couple BIF samples with the D610. The first is not a magazine shot, I know. But, it was a looooong shot and heavily-cropped, and on a completely overcast day. In fact, it started raining shortly after this shot. It was taken with the regular (but my semi-beloved) 70-300 VR. A better lens and a better photographer may have provided better results. The second was with the same D610 and 70-300, but much better conditions. Good light, and closer range. I'm sure the D750 is better, but is the D610 good enough???
Wow! How much crop? I'm back and forth on the 750 and 610 myself. I like the high ISO performance of the 750, but I always try and keep it 800 or under anyway, if I can. So mostly a moot point. Obviously the 610 can track.
Below is the eagle image out of the camera and before the big crop. After shooting with the D750 for a while now, what really gives it the nod over the D610 is how it auto-focuses in the dark. I shot a couple of outdoor weddings at sunset, and both cameras hummed right along. But with my light stands set up for post-ceremony group shots and the sun down to the point that the view finder barely makes out black silhouettes of people, the D610 AF did finally raise the white flag, and the D750 kept right on going. The 'Group' option within the D750's AF system is really nice too, and seems to work better for me when I stop down and go for birds in flight or shoot sports. And yes, it does handle high ISOs a little better too. If you say you never go above ISO 800 and you rarely shoot in extremely dark conditions, I would be tempted to save some money and go D610. IQ will be identical. If cash doesn't matter, the D750 is one sweet rig for sure. I love the deeper grip too.