Terra
Damage from IS idol will be about 118 * number of IS ticks (IS glyph, not tier bonus).
Damage from SF idol will be about 263 * (SF non crit + 2.09 * SF crit)
If you manage only 2 SF (one crit and one non-crit) every 12s, and maintain 100% IS uptime, the SF idol still wins.
IS Idol can be better on fights with enough movement, but with anything close to a PW-style fight, the SF idol will be much better.
Edit:
Graylo says that the idols actually increase base damage, not spellpower. Since both IS and SF have coefficients of 1.2, both of my numbers above are 20% too large. For comparing the two idols against each other, that is ok, just don't count on their values in an absolute sense.