i already get a free amex earning 1.5 QF points per dollar and have a separate MC and Visa each earning 1 QF point per dollar (both free as well), so no interest. Add to the fact that CBA is the worst customer service out of any bank in Aus, a really low bar, and it's a no brainer.
The worst customer service? Never had a problem and have been banking with them for the last 18 years.