I got my 7 Plus on April 6 and it's a tremendous upgrade from the 6s Plus.
I'd say go for it =) if you ever want the new one, in September, you could aways sell your 5 months old 7 Plus for a very good price! It'll still be popular among people who want a recent iPhone but can't afford the latest.
I have a 6s and really want the larger size of the 7 Plus. Am I crazy? Next phone is expected in September.
I'd consider a 6s Plus but really want the depth effect as well, which it doesn't have.
Why would you wait 5 1/2 months if you need/want one now. There is bargains to be had now no need to pay retail, i got a New sealed 7 plus 256 gb for $600 below retail and a New sealed iPad pro 9.7 for $370 below, all in the last month.
Cash is King.
At this time you're just too late to get the 7 Plus. Your 6s is a quite recent iPhone and I don't think the benefits of an iPhone 7 are worth the upgrade from a 6s.
I'm sure you can hold on a little longer to your 6s. In the end, what is waiting 5 more months in iPhone land?
If you do get a 7 Plus now, you will regret it when the newer iPhones come out.