This can easily be settled by comparing various combinations of weapons and eidolons using my Toolbox and seeing which combination maximizes HP and ATK. I'm not doing this because it's not my team or element and the answer doesn't interest me, but those with a vested interest (i.e., the OP) should do so. But in general, Defender SSRs have such a large increase in base stats that it nearly offsets the loss in damage from assault.
My suggestion to keep Echidna is based on an analysis of Fafnir's stat scaling using stats listed in the JP wiki (these might be different from Fafnir's stats in the Nutaku version). This may or may not be applicable to the current situation, which is why I've made a calculator available for players to determine what's best in their individual situations. When it comes to optimizing one's team, there's no substitute for running the numbers oneself because every team configuration is different.