The MSI MAG B550 TOMAHAWK wins in 7 out of 13 comparable specs, with 5 ties. Its biggest advantage is Memory Slots (100% better).
PRO B660M-A WIFI DDR4 wins in 1 spec (MSRP: 25% better).
Recommendation
Choose MAG B550 TOMAHAWK if you want the best overall performance — especially memory slots.
Choose PRO B660M-A WIFI DDR4 if msrp matters more to you.
