I use a Nikon D3, Its great for what I do and need it for, if you don't require shooting above ISO 4000 a lot then you mite aswell safe your money and go for the D3, I have shot tons of wildlife at ISO 2500/3200/4000 and got clean images from them with a bit of Noise Reduction
CLICK HERE for a shot at ISO 4000, Check out my
Flickr if you want to see some high ISO shots from it, The D3s will give you crystal clean images upto 12800 ISO, Apart from that everything is the same apart from the D3s has 720p Movie which I doubt anyone hardly uses...The D3s also has a deeper buffer but your still get a great burst rate out of the D3 at 9fps or 11fps in DX mode IMHO, The D3s also has sensor cleaning in camera which the D3 doesn't, I personally use a rocket blower everytime I switch bodies so that doesn't bother me to much, According to Snapsort the D3 beats the D3 on having slightly better battery life and 0.2EV better dynamic range and shutter lag only being 88ms compared to the D3s 197ms, All in all if you don't need high ISO capabilities then just get a D3 and put that extra £1000 towards glass IMHO