Hi there,
The Dell 2412M and U2311HM are very nice monitors, so either one is a good buy.
As for the 0.6ms, that would be input lag vs CRT of the U2312HM. This is a very good figure, but to be fair the U2412M offers an input lag 9.4ms which is also very good (under one frame). As for response time, from the TFTcentral tests they say they is pretty much on-par, though the U2312HM is slightly better.
However, if you plan to run an Xbox on one of these monitors then you would probably best with the U2312HM - due to it's 16:9 aspect ratio and 1920x1080 resolution. The U2412M is better for desktop use and (many would say) PC gaming - but on Xbox gaming and watching widescreen movies a 16:9 monitor is a better bet. Also, since the U2412M doesn't do 1:1 pixel mapping then it will vertically stretch a 16:9 image output from a console, which isn't ideal.
As for the anti-glare coating, this is basically a standard thing with Dell monitors which are in reality professional/office monitors. So they don't have the shiny surfaces and vibrancy of competing consumer focused monitors - but instead the anti-glare is very effective and the image quality is still very nice.