I know its a tricky situation for you given the time constraints, but as long as you understand what advantages the 6 has
Personally I like the design, its very comfortable to hold (rounded rather than square), some people like Apple Pay and use it religiously (I am indifferent to it), WiFi ac is very nice and fast (provided you have an ac router), and the larger screen size certainly helps especially with typing I found. There are also other small tweaks like to the Touch ID and Camera
Its a great phone and you'll be getting extra storage capacity too which seems to be important to you
However - it seems you aren't able to wait for the 6S, which is really something you should try to consider. My main reasons for that are 1) 6S is around the corner and is likely to be a substantial upgrade to the 6
2) with the 6, you should know that you're getting a phone that is very similar in power and speed to the 5s; what you already have. The 5s and 6 aren't too different in CPU or GPU speeds, they have the same amount of RAM, and when it comes to performance you'd be hard pressed to find a difference between the two in real time. So if speed and power of a device is important to you, which it is to a lot of people considering that is what limits how fast or slow a phone performs tasks, I would take this into consideration as well because it seems not many people are aware of how similar the 5s and 6 are in power.
The 6s will likely be an improvement over these two devices, so that is something to consider when you're using it daily and if you plan to use it for a few years. I guess my thinking is - you have a 5s, and this close to the 6s launch you're getting a phone that is almost identical in performance to the 5s. If you value other features more - like the design and screen size, you'll be getting those as well by waiting a month for the 6s. But if you are on a short time table and can only take advantage of a deal right now then, do what you feel is right for you.
