I know nothing other than that 7xxx is better than 6xxx.
Sadly, this is not the case.
from wikipedia (still learning this blank board, so copypasta is faster)
6000 series are alloyed with magnesium and silicon, are easy to machine, and can be precipitation hardened, but not to the high strengths that 2000 and 7000 can reach.
7000 series are alloyed with zinc, and can be precipitation hardened to the highest strengths of any aluminium alloy.
7000 is just harder and more brittle IF it's hardened.
6061 and 7075 are common.
6061 density of 2.7 g/cm^3, yields at 240 MPa minimum (for T6 )
7075 density of 2.81 g/cm^3, yields at 430 MPa minimum (for T6 ) and is significantly more expensive to machine. un-heat treated 7075 is only 275 MPa max though.