We've written several times about the ongoing comeback of IPS panel technology. In a nutshell, IPS tends to offer visibly superior image quality to other panel types, and although IPS monitors used to carry hefty premiums, they're now much more affordable and widespread.
These two displays have 1920x1080 resolutions, DisplayPort inputs, 178° viewing angles, sRGB color, and pretty similar specifications in general, although the 2311H does have a lower luminosity rating than its 21.5" sibling. Dell actually lists IPS technology as the first bullet point on the product page for each monitor, so there's no chance of a bait-and-switch.
As icing on the cake, Dell covers the U2211H and U2311H with a zero-bright-dot warranty, which means you should be able to send either back and get a replacement if you find so much as a single stuck pixel.