I don’t have a D3s but the D4 low light focusing is amazing. It is a faster focusing camera than the D3s. If you are going to crop, you should check out the D800. What are your priorities? I got the D4 mainly for motor racing and the D800 is just not fast enough to capture a race car going across the front straight at over 160mph, sometime close to 190mph at the braking zone. If you get a good deal for trade and don’t have to go through the trouble of selling it yourself, go for it. You'll be happy with the D4.