CrossFireX is 4-way Crossfire. Are you planning on Crossfiring 2x HD5970's?
4x HD5870 is better than 2x HD5970 because the HD5970 was downclocked to stay within the power realm of 300Watts.
For just plain Crossfire, again, the HD5870 is better. However, if I had the money and I were you, I'd buy an HD5970 now, and about a year later, crossfire another one. The card will be significantly cheaper.