Personally I'd much rather use a 7700x to not have to deal with asymmetrical cores. They perform close enough otherwise, I think with the 12900k having a minor edge in some tasks, including acting as a space heater.
Also, 7700x buyers can expect CPU upgrades for a generation or two on the same motherboard. Intel's 14000 series (13000 part 2) is the end of the line CPU upgrade for the 12000 buyers.