Linux, the 3 cell one (the one i got it with a 2 cell has about 4 hours left). I get about 9 hours with full light and wifi and about 14 with ethernet & slightly dimmed light. Obviously linux (also Arch), i dont think Windows can be optimized for such battery life. And in my experience Arch also beats Ubuntu. At least without a lot of hacking.
How much battery time are you getting?
And which battery do you use?
And are you running Linux or Windows?
(I'm considering a T420 right now)