If the reason is storage, then yes, it's probably in your interest to upgrade now and I'd recommend doing so.
The 6S/6S+ is a nice jump from the 6/6+, especially with the 2GB of RAM - makes a huge difference in day-to-day usage. But, you're getting closer to what would be a mid-cycle update, which bothers some people. It wouldn't bother me as I tend to like S-cycle releases anyways, and if I needed a new iPhone now, I'd get one (if it was July or August, though, not so much - I'd wait if at all possible). If you do bite the bullet and upgrade now, the current 6S/6S+ would continue to serve you well into the future and whatever S-cycle comes out in the Fall of 2017 though...
In the alternative to using your jump upgrade now, have you looked into purchasing a used 6+/6S+ on sites like swappa or Glyde? On swappa right now (10:00 a.m. Eastern on March 7), the low price on a T-Mobile specific 64GB 6+ is $510 and $630 on a 64GB 6S+. You would have to pay in full, but you could sell your current 6+ to offset some of the cost now. This option could get you the storage you need now while reserving your Jump for the 7/7+ this Fall.
As to the possibility of a 7/7+, I'd say it is a near certainty that there will be a 7+ - I don't see Apple moving away from the phablet territory given the popularity of the 6+/6S+.