Should I buy a WP8.5 this year, or wait til WP9 next?

Hey this is sort of almost obvious question. But, should I get the newer Windows Phone this year?, or wait til next year? Really, this is kinda about the tick-tock schedule. If last year was the tick, then this year would be the tock, right?

I'm on T-Mobile, and generally my contract ends in like December. But, I stop paying for my phone in like August. So, i can upgrade whenever i want really. And I'm really liking this new pricing they have. ~ But, should I jump on the tock-schedule this year, or wait til next for the new Tick?

I'd rather a Microsoft Surface Phone, but it really seems more and more that this isn't the year for that.