What do I get if I get Poke-Bank (besides the ability to transfer Pokemon up from older games), and, are there any alternatives?
There's usually some kind of promotional event going on that gives you a free Pokémon. Back when it was released, it was a free Celebi.
Right now, there seems to be a promotional event running for three free Pokémon by using Pokébank, namely:
They all have a Hidden Ability. It looks like it will be running from Feb 27 to Nov 30 2015, so you have plenty of time to get them. While there's no confirmation, I daresay there will be another promotional Pokémon after that. So far, like Event Pokémon in the games, you don't have to be a "new" player to get these. If you've been playing since the start, you'll have access to all of them.
Other than that, and the ability to transfer Pokémon through the internet, there's the added benefit of having a 3000-space Pokémon storage, should you ever have that many Pokémon.