Depends on the cards imo. It's better to go with one high-end card than 2 mid-range ones. (Better cooling, less power consumption, etc)
However, if you already have a high end card like a 7950/7970 or a GTX670/680 there's honestly not much point in upgrading from the 7950 to a 7970 or from a 670 to a 680. You'd be better off buy a second card and Crossfire/SLI them