I really look at both monitors and the Asus has the better quality of the two but I have a dell monitor and it is a 23" but not as good as the one you linked us too. Ok I will give you the pros and cons.
The Asus has a bigger screen has better contrast ratio then the dell and it probably is lighter and it is for sure thiner. It is easy to connect your cables in the back and It probably has a better picture.
The dell you have to plug in the cables from under neath which is I don't like. But what I do like is the usb ports so if you have wireless stuff you can plug them in to it or you don't have to run all of your other stuff to the back of your PC.
I like the fact you can rotate it up on its side it will do go up and down which my dell monitor won't do any of that. My monitor is the ST2310 and It is going on 2 and a half years without any problems. Knock on wood.
Plus dell has a good warranty and good service. I don't know about Asus. If I had to pick I would get the dell. But it is up to you. Just remember the Asus has a better picture and spec's than the dell.
I hope this has help a little and good luck to you.
The Dell has superior picture quality and contrast being an IPS panel. The one downside to IPS panels are the slower response times. However, it is extremely unlikely that you will be able to notice and difference in response times. If you like to watch movies and games, go with the dell. If you consider yourself a hardcore gamer and need the absolute fastest response times, get the ASUS. (I do recommend getting the U2412M for a 16:10 aspect ration as long as you dont need HDMI.)