I think it's impossible to predict the market / the direction of things. For a while node.js was a big thing, these days I don't hear as much buzz about it. Then there was the period of buzz around nosql. I say just do what interests you the most, don't learn what you might think is the next big thing. There's also nothing wrong with continuing down the path of iOS. You can be mediocre at a bunch of languages or an evangelist at one.