The one where the chips used in those mainframes have the edge on those used in servers in some important ways - more cache, higher cache bandwidth, lower cache

The one where the chips used in those mainframes have the edge on those used in servers in some important ways - more cache, higher cache bandwidth, lower cache latency. Those developments will eventually make their way into server chips and with a bit of luck IBM (et al) will have developed something else by that time which will eventually enter mainstream.