They do not differ, except for elemental resistances. For example, Shiva is highly resistant to Paralysis and Blizzard Spells, and Diabolos is highly resistant to Sleep. Celestial Avatars and Diabolos count as BLMs, and have MAB traits at the same level as BLMs.
Carbuncle has less HP than all the other avatars, and is a WHM main job (and has Auto-Regen as a result). Carbuncle also hits the weakest because of this.
Fenrir has a lower delay than the other avatars, and also hits harder.
Kegsay - 85SMN/85BLU/85SCH
Goldsmithing-60, Cooking-61, Synergy-60
Shaded Specs Obtained 20 June 2005 http://kegsay.livejournal.com/