The question, "When is a Gatcha eidolon better than an MLB event SSR as primary eidolon?" has come up several times lately, so I added an Eidolon Comparison tool to the Constitutive Effects Damage Calculator to answer it. The answer for Fafnir is: a slight damage increase at LB1 (~2%), and a significant increase at LB2 (~4.5%), both at the cost of some lost HP. At LB0, the loss of stats makes the Fafnir-type eidolons undesirable as primary eidolons.

I also upgraded the Weapon Comparison tool with the option to factor in the weapons' Burst modifiers (when equipped as a primary weapon).