The Dell monitors have the standard DisplayPort connector (they actually use DisplayPort natively so you definitely want to use DisplayPort instead of HDMI or DVI), but Macs require the mini DisplayPort connector. The protocol being used is the exact same thing (DisplayPort), only the connectors are different which is why you only need a cable with a standard DisplayPort connector on one end and a mini DisplayPort connector on the other end. DisplayPort (the protocol, not the connector) can output resolutions of 2560x1440 or 2560x1600 without any problems no matter what connector you use.
There is only one difference: the mini DisplayPort connector does not support audio which the standard DisplayConnector does. I don't really consider that to be a problem because it is used for computer displays which mostly (as in 99% of the displays) lack audio. The ones who have audio use a 3,5mm jack for that.
Dell displays are really great because they have great TFT panels and they use DisplayPort among DVI and HDMI (aka multipurpose displays!). The only downside is the slowness of the display when switching outputs and when waking from standby but not all of them have this problem (the 3008WFP does 🙁).