Fafnir also gives a defense buff. Stacking on another +12% defense from the summon slot's a pretty nice addition with the +12% atk. It's like throwing on another upgraded Rising Heart.

Typhon's listed as a +10% atk buff. +12% vs +10% atk isn't a big difference, so Typhon lasting one more turn's more useful outside of 'scraping together every little bit of attack for full burst' situations. So, point to Typhon there, but that def buff gives the edge to Fafnir for me.