More accurate description below:
6000-series is aluminum alloyed mainly with magnesium, and tends to be a bit more ductile (for instance, it would be more apt to wilt or buckle than to snap, compared to 7000-series).
7000-series aluminum is alloyed primarily with zinc. It tends to be harder and stronger but perhaps more brittle. Excellent material for chainrings due to its hardness, too.
Both materials can be found at the lower end and the upper end.
It depends more on the frame manufacturer as to the quality of design. Factors such as double butting, weld quality and tube shape come into play here and are being continually developed.