Im not sure about the dell,though ive heard its a very nice monitor but for me the extra smoothness and ability to show 120 "true" FPS makes it the one to get for first person shooters certainly.Although i had some issues with the ACER (you may get one thats fine) as witnessed in these threads
http://forums.overclockers.co.uk/showthread.php?t=18154969 and
http://forums.overclockers.co.uk/showthread.php?t=18165523
I would strongly recommend the ASUS VG236H.Im very happy with mine and IMHO its the better of the two.Both are 1920x1080 the acer being MARGINALLY bigger at 23.6 vs 23 though its not that noticeable at all.The only other thing with the asus at the moment is that as far as i know it comes with the 3d glasses also unfortunatly(though i believe they are gonna sell one without it called the VG236HE).I paid £375 for mine,but i have a buyer lined up for the glasses so the true cost of the monitor to me has been around the £300 mark.
Yours
Dash