Prestige 3 Kerrigan is a sidegrade. She’s better vs air, but has less overall “I can solo this entire mission” capability than she does in her regular state. The increase to Assimilation Aura is the big benefit here.
I don’t like Prestige 3 Artanis and I have no intention of grinding for it. Guardian Shell is by far the best thing Artanis has going for him as a CO, and losing it for what that Prestige gives you does not seem worth it at all.