I feel that an upgrade would be worth it. Here are my reasons as I feel they have not been discussed. This is since you are going from a 6plus to a 7 plus. If yours is last year's model, I would not recommend an upgrade.
I used to have a problem with safari reloading pages all the time due to the smallish RAM on the iphone 6plus. I upgraded to the 6s plus and no longer have this problem. If you load multiple tabs, these can be bad for those with low data plan on the old phone. The higher RAM prevents this.
The force touch is a godsend for me. I edit alot of text on the go and the ability to use the cursor to go anywhere on the screen i want easily saves me a whole lot of time. Various apps also have shortcuts based on force touch.
Going from the 6plus to a 7plus, you will find everything moves faster and you will also have the option of a higher storage base.
The camera quality is much better.
Since you already made some money flipping, why not buy a 7plus (in a color and size of your liking without the urgency) and sell the 6plus to further "subsidize" your new phone. If you sell the 6 plus next year, the value would drop even further.
My 6 plus is 2 years old but still going strong. Have alr recon but sold my 7 plus for a good price.
Tempted to buy a 7 plus from apple store at rrp. Those who change from 6 plus to 7 plus is there a huge diff ? The most intensive thing i do is load many multiple safari tabs and YouTube / Netflix
