The AX and HX are different series. They started some time ago with some differences.
The HX was semi modular. The HXi is fully modular. I think the AX was as well.
The HX is deeper than the AX. This is still true for the HXi.
The HX has a larger fan. This is still the same.
The new HXi adds corsair link support if that is something you wanted. They both offer a fanless mode for low loads(when the HX was released it did not have this feature, it was added to the HXi series).
The are both platinum certified meaning the generate less heat and waster less energy to make the same amount of power as some other power supplies on the market.
I would go with whatever fits your needs(for some users the deeper power supply does not fit as nice.).