The big difference between these monitors is the Panel Type. the BenQ has a VA panel, while the DELL has an IPS panel. VA panels have decent color accuracy, but what they are really known for is their better black values and much better contrast than any other panel type.
IPS on the other hand, has the best Color Reproduction out of all the panels, and usually has the best color calibration settings built in to the monitor. they also have the best viewing angle and much better real world response time (VA panels are known for some ghosting effects).
Over all, the IPS is probably the better choice, although they can run a little more expensive.