I'm currently using the 2014 notes and I managed to have a look at the 2016 ones. And when I say look, I mean, I flipped through the books for about a minute each.
There's not much of a difference. It's formatted slightly differently such that there are more lists in bullets rather than long sentences, but I couldn't see any major differences.
My 2 cents:
1. If buying 2016 is going to cost you a lot of money and you already have 2014 - Don't buy
2. If you haven't started your prep and you're thinking of which one, well, that's up to you depending on how much money you want to spend.
3. If you're half way through prep using 2014, quit worrying about 2016 and move on. The mainstay of your prep is going to be Uworld anyway. There't no point wasting time on minute details which you will pick up from elsewhere during the course of your study.