Well, it does not directly answer his question, but offers an alternate solution. I was suggesting that if there is nothing wrong with his current Mac, then why get rid of it. I am sure that a good number of users, do not really need any new features that may appear in a new release of a Mac. I don't plan to change mine, unless it goes really bad...After the hinge replacement, it has no big issues that bother me. I know the newer ones might be a little faster, they screen resolution of the display may be a little better...but none of that bothers me.
But, here to answer the question more precisely. I personally would wait until High Sierra is released, as if a new model is released, it will likely be at the same time as High Sierra is released. If I were planning to buy a new one, and was not pressed for urgency (laptop stolen, fell out of airplane into Salt Lake, or other random catastrophic event). I would plan to utilize the Mac for as long as possible, so waiting a few more weeks or a month, to possibly have a better model for a number of years would be a great thing to me. As far as the "beats" go, I am guessing you are referring a deal that gives you a pair of headphones. I don't use headphones, and keep the computer muted at most times, so that really isn't a thing for me, and would not affect my decision at all.