The AX370-GAMING 5 is completely the same as the AX370-GAMING K7 except for one difference, the K7 comes with the Tube B-CLOCK chip which facilitates a wider range of BCLK frequencies.
It's the premium features like a better VRM or support for BCLK overclocking that add value.
This may come as a rude shock to enthusiasts, who previously would also overclock a processor with a locked multiplier by playing with the Base Clock frequency (BCLK).