Like Shoryuken eluded to in his sample build, you could pretty much eliminate microstuttering by having 3 cards. Tom's did a bunch of benchmarks with high end cards a while back to analyze microstuttering. 2x any card has microstuttering. Single and triple card set-ups have almost none.
But if you're just interested about the 690 vs 7970xfire, then go with the 7970xfire for performance.