Depends on the version you're playing as well.
E.g. in the latest HttT.. For Byzantium, the easiest way to expansion CB/BB wise is east, but needs missionaries. For the Ottomans, that road doesn't need (m)any missionaries, but CBs are harder to come by. Relations with the big neighbour up north are also quite different.. If it's Russia, Byzantium has an easier time staying friendly. If it's the Golden Horde, the Ottomans.
Byzantium will never be the target of a crusade (doesn't mean the westerners won't jump on you, mind), the Ottomans are quite likely to be.
E.g. in the latest HttT.. For Byzantium, the easiest way to expansion CB/BB wise is east, but needs missionaries. For the Ottomans, that road doesn't need (m)any missionaries, but CBs are harder to come by. Relations with the big neighbour up north are also quite different.. If it's Russia, Byzantium has an easier time staying friendly. If it's the Golden Horde, the Ottomans.
Byzantium will never be the target of a crusade (doesn't mean the westerners won't jump on you, mind), the Ottomans are quite likely to be.