Most bindings are so close to each other that just a little bit of snow packed on your board will completely negate any weight savings. Like others have said, just find a pair that you like and feel comfortable riding.
I saw a thread from another forum a last year that had all the bindings weighed and the contrabands came in the lightest. I don't know if that is still true since they have changed since the first ones a good bit.
There may not be a single lightest binding. With the large size range of bindings the lightest for a size size 7 boot could be different then a size 8 boot.