The OP is asking what would be the best career move, not what would be the most fun.
King Air flying can be fun, and even quite satisfying. But it is a 50 year old design and it is all too easy to end up locked into that type of operation.
Usually salary is at least partly proportional to the revenue generated by the equipment.
Hence, for career, go A320 FO, build seniority, get command.
Approaching retirement is time enough to seek out the fun jobs.