With all else equal, 6990.
1) resale value if you ever want to dump it.
2) higher hashrate on the 6990, a quick look at the hardware comparison shows ~100MH in favor of the 6990
~600 (5970) vs ~700 (6990) at stock clocks
I haven't looked at power consumption numbers to say if that makes enough of a difference to make up for 100MH, but it'd have to be significant.
They also are pretty even at top overclock on that site. Honestly these cards aren't too great unless you get them for around 500(which you won't) so in order to even get close to a good megahash per dollar value you have to overclock. 1) resale value if you ever want to dump it.
2) higher hashrate on the 6990, a quick look at the hardware comparison shows ~100MH in favor of the 6990
~600 (5970) vs ~700 (6990) at stock clocks
I haven't looked at power consumption numbers to say if that makes enough of a difference to make up for 100MH, but it'd have to be significant.