Both these statements are true. Two mid-range video cards in SLI/Crossfire will usually yield better performance at a lower/equivalent price, UNDER IDEAL CONDITIONS.
Most of the time it is better to use a single high-end GPU, as multi-gpu configurations have some issues: microstutter, more power/heat, shared memory, some games simply won't take advantage of multiple GPU, difficulty of upgrading, etc...
If you know that, for the games and resolution that you plan to play, a dual GPU setup would work, it might be more cost effective to go that way if your current hardware can accommodate multiple cards.