The two cheaper models, U2211H and U2311H, have mechanic buttons lined up vertically along the right edge of the case. They click slightly when pressed. The Power button is the bottom one. It has a soft LED indicator which is blue at work and yellow in sleep mode. Take note that the buttons have no labels because their functions can be reprogrammed in the monitor's menu.
The more expensive models in the series differ a little. Their buttons are all touch-sensitive and can be found by the square pimples of their blue LED indicators. The LEDs go on when your finger gets close to a button (without actually touching it), so you can easily hit the desired button even in full darkness.
I didn’t find any difference, other than aesthetic, between the touch-sensitive and mechanical buttons. Both types are equally handy and work properly. The touch-sensitive type look more exciting with their LEDs, but that’s the only advantage I can see about them.
The more significant difference between the models is the number of inputs and outputs.
The U2211H and the U2311H only have a couple of USB ports on the left panel.
The three other models additionally have a Secure Digital reader there. I personally don’t find these features handy, though. Having monitors with side USB ports both at home and at work, I’ve never really learned to use those connectors. It just seems inconvenient for me to fumble for them or to turn the monitor around. I guess such USB ports may be useful for a permanently attached device, like a receiver of a wireless mouse, but the USB connectors of my system case are far easier to access when I want to connect a flash drive, MP3-player or something like that.
At the back panel the U2211H and U2311H have the following: a DisplayPort, a DVI connector, a D-Sub connector, connectors for an optional speaker module, one USB input, and two USB outputs which you may want to use for your keyboard and mouse.
The Dell U2410 offers two DVIs, a DisplayPort, an HDMI and a D-Sub connector, and two video inputs (component and composite). If you don’t have the optional speaker module, you can use the audio output for your headphones. The monitor doesn’t have a dedicated audio input. It receives audio via HDMI.
The 27-inch model adds more audio outputs. It has three rather than one stereo connector so you can connect a multichannel speaker system. There is still no analog audio input, so the monitor can only receive audio via HDMI or DisplayPort in PCM format (it lacks a Dolby decoder).
The top-of-the-line U3011H model adds a second HDMI to this very extensive selection of video interfaces. It lacks a composite video input but I don’t think a 30-inch monitor needs that interface with its rather low signal quality.
So, while the difference between the touch-sensitive and mechanical buttons is purely aesthetic, the selection of interfaces depends on how expensive the particular model is. The number of video inputs varies from three in the junior models to as many as seven in the senior one. Each model has a DisplayPort which is a rather new interface.