you've kinda answered your own question really haven't you?
the positives of doing it yourself are that you get unlimited for free and probably have more control (although a push notification is a push notification is a push notification... how much control do you really need).
the positives of urban airship are that you save time/effort/resources. You don't have to maintain anything or fix any bugs.
I'm assuming if you send over a million push notifications you can probably monetise your app and thus pay for the service anyway. Remember a million is a huge amount. You can send 100 people 10,000 push notifications.
