Hmmmmmm... that's the thing with technology - always something better around the corner.I suppose when the iPhone 7 comes out, they are going to ask "Should I buy 7 now or wait until the 7S comes out". I'd say just buy it now, and be happy with what you have. I suppose if you are a type of person who needs that much space, then you can play the waiting game.
The 6S Plus would be cheaper once they release the 7/7S. And yeah, time will tell. I hope 32GB will be the base (which is around 25GB).The 6S and 6S+ are the first really, really good iPhones for three years in my humble opinion. It's a good time to upgrade.
The iPhone 7 will be a form factor change and still have 2GB of RAM, plus an incremental processor & camera upgrade. It's doubtful that 256GB will be on offer, more likely that 32GB will be the base.
I'd be upgrading for the storage difference in the 6S Plus. But I'm forced to buy the 128GB 6S Plus if I want the highest of storage.Since your sig says that you currently have a 16GB 5S, then the upgrade is certainly worth it. However, as you have not been tempted so far, means you can potentially hang on for another year with your current phone. Next year, he 1GB of RAM in the 5S will really start creaking for the cutting edge apps. A paid off phone is better than spending more money just for the sake of it.
Well if you feel that you are being "forced" into he 128GB now, then spending another year with the 16GB is not an option at all. Then get the 256GB whenever Apple releases that capacity.I'd be upgrading for the storage difference in the 6S Plus. But I'm forced to buy the 128GB 6S Plus if I want the highest of storage.