The 8ms number represents the original revision A00 model. I believe the newer revisions now feature an average response time of around 6ms~.
I'm not sure why everyone is going bananas over response time lately. I've only ever noticed any slight delay on a monitor rated at an average response time of 14ms.
I think the extra price is worth it. You'll be getting a monitor that has great color calibration options and an amazing stand which matters more than you might think. Not to mention the U2412M is VESA compatible which is a big plus if you want to wall mount your monitor in the future.