Biggest difference will be bandwidth. AX6000 = theoretical 6000Mbps (2.4Ghz (1,200Mbps) + 5Ghz (2,400Mbps) + 5Ghz backhaul (2,400Mbps)). AX4200 = theoretical 4,200Mbps (2.4Ghz (600Mbps) + 5Ghz (1,200Mbps) + 5Ghz backhaul (2,400Mbps)). The backhaul radio is a dedicated band that connects the two Orbi devices wirelessly and shouldn't be considered in the bandwidth equation but "advertising"... So realistically AX6000 theoretical max bandwidth is 3,600Mbps with the AX4200's is 1,800Mbps.