It's hard to say 'demons are better than elementals' because there are so many different kinds of demons and elementals.  If I summon a Sobeksis and you summon an angyalka, the Sobeksis is going to come out ahead in the fight, on account of being an eight foot tall crocodile-monster.<br/><br/>The differences, though are this:<br/><br/>Elementals are easier to summon and to bind; they are also easier to banish.<br/>Elementals play a part in the heavenly bureaucracy, and are protected by Heaven.  You can't abuse them without pissing off the gods.<br/>Elemental summons are shorter: one month (28 days) vs a year and a day (426 days) when bound into servitude, and a year and a day (426 days) vs 'as long as it takes' when bound to a task<br/>Powerful elementals can refuse summons entirely, sending servants in their place<br/><br/>Demons tend to have specific purposes whereas elementals tend to be weird and just kind of pop up and do their own thing, so it can be easier to find the right kind of demon for a given task than an elemental (considering that lots of demon breeds were developed specifically to fill some sort of function, while elementals just sort of pop up on their own).