This sub-forum has very little traffic.
Look at all the threads on in this section and note the number of replies, then compare it to the other sections like Graphics & Displays (don't know why "Display" is there since there no sub-forum for it), or CPU & Components. Note the vast difference in the number of replies.
The only active posters in this sub-section are Rofl_My_Waffle and myself. Adriod pops in every now and then...
Now regarding your question...
You need to figure out what is more important to you. Higher resolution? 3D gaming?
Generally speaking, "Higher Resolution" allows you to see more on the screen (it also depends on how the game is programmed) so that it is possible you can react faster. But "seeing more on the screen" can also lead to information overload to the gamer which means slower reaction time. It depends on the gamer.
3D gaming... never seen a 3D game. I have seen the movie Avatar and the 3D effect of the movie wore off on my in less than 30 minutes... In other words, I did not find the 3D effect impressive.
My choice would be the Dell U2711 because I really don't care about 3D gaming. More importantly, it uses better panel technology (H-IPS) than the Alienware (TN) for color accruacy and viewing angle purposes. The higher resolution also means I have "more space" to work with; mutliple windows openned. Also good for watching movies...
However, the Dell U2711 has a few negatives as far as gamers are concerned...
1. Price - Most "hardcore gamers" prefer to spend money on the video card than the monitor. There's a $600 difference between the Dell and Alienware. This mostly due to higher manufacturing costs for H-IPS panels and more materials need for a 2560 x 1440 resolution monitor.
2. Response time is 6ms for the Dell and 3ms for the Alienware. Not a big difference, some gamers would gripe about the 6ms.
3. The input lag of the Dell U2711 is considered a bit high for most hardcore gamers' taste. It's input lag has been tested to be about 30ms. That is nearly a two frame lag; 1 frame of lag = 16.67ms. Most hardcore games would want this to be less than 10ms. I don't know what the input lag of the Alienware is; never read a review on it.
In case you do not know, input lag is amount of the between an action you are performing on the keyboard / mouse or a gamepad and when it appears on the screen.