The Ultrasharp is an inch larger and at a £20 higher price of the Asus and has 2 HDMI ports, 2 Display ports (1 of these being a mini port too) and 4 port USB hub.It features a very sleek and slim design with an extremely thin bezel which looks lovely.
The 23" Asus is slightly cheaper and has 2 HDMI, display port, DVI-D, VGA, Headphone port and 4 USB ports. It has a slightly plasticy look to to stand and bezel but looks like a solid choice. It also has a built in grid feature allowing image editing, 3D creators etc. a handy grid to use off the fly.
The decider is the response time. Asus has 5/6ms and the Dell has an 8ms. I know IPS panels aren't meant to be as fast as a TN panel which is fine however, i will be using the screen for my Xbox 360 and my PC featuring a R9 280X Toxic from Sapphire.
response time claims are usually fictitous , or based on tests that never occur in the real world . I would not use this as a point to make a decision on
If you cant actually get to see the monitors in action then I suggest you google for reviews