Grand, the 5970 is literally two ATI GPUs on one card, it is also the ATI flagship enthusiast card, the best of the best, it currently commands the highest performance and therefore, premium on price. The ATI card also has the wider memory interface, 384-bit vs 512-bit which can make a huge difference in performance. It's framebuffer (2gb x2 = 4gb) is also considerably larger even though it is essentially two cards in one.
Sadly, even in spite of the piggish power demands and high heat output of the Fermi, the 5970 still beats in such demands I suspect, however again you must take into account this card is basically two ATI cards already running Crossfire(intergrated) and it will run as such. Providing you have the right monitors and number of them, you can have a VERY nice Eyefinity setup for Freelancer or whatever else you wanna play on it.
If you can't swing the ATI card, by all means go for the Nvidia, I've used both over the past years and each will serve you well, just make sure you have proper cooling for your case and a power supply to meet AND exceed the demanding needs of these modern, high-end video cards.