D3S vrs D5100
IMO, if you're happy with the images that you're getting from the d5100, what possible difference would it make to have the opinions of others on the relative ISO performance to the d3s?
The other differences between the 2 cameras are much more significant than the differences in ISO up to 1600-3200, IMO. If you would prefer to shoot with a smaller, lighter kit, it's likely that you'd enjoy your photography a lot more if that is what you used.
That's just my 2¢, FWIW.
good luck with your decision.
my gallery of so-so photos