The 6990 is generally more powerful than the 7970. It is a dual-GPU card so it relies on CrossFire to perform well. The 6990 is still AMD's single most powerful card but they are very limited in supply and expensive.
The 7950 looks like the better high-end value. They sell for $460 on Newegg USA vs. $550 for the 7970. If you are interested, a 7950 with a good overclock comes pretty close to a 7970 at stock clocks. Check out: http://www.tomshardware.com/reviews/radeon-hd-7950-overclock-crossfire-benchmark,3123.html
Rockdpm - I see a 7970 for $550. Where are you getting the $200 difference from?