I don't really know difference between those in term of considering to buy one. Which one is more worth? iMac can be as display monitor?
The 27" 2011 model iMac is a computer and can be used as a display. The display is only that and you will need to buy computer in addition to that.
So if you can find a 27" 2011 iMac that can be used as an external display, get it.
Do you mean the broken iMac that can still be used as an external display?