Wouldn't you be better to try and jump to the x?  Though if you're a 6 (not 6s) you're still making 4 jumps to an 8  I thought they were pretty similar in terms of CPU etc.  But the 8 didn't have a 128gb option.

It pretty much comes down to how much storage you want,

Apple in its consummate slickness don't give you an "apples to apples" option ...  ha ha

you have

32GB 7+ $1069

64GB 8+ $1299

128GB 7+ 1269

256GB 8+ $1599

I went from a 6+ to an 8+ 64gb and don't regret it.

I would have liked to try the X, but to be honest, the screen is too small (I really like the big display).

In a year or 2 I'll be going for the Xs+ or whatever they have then.

I saw the Xs+ in PB tech and I really like it, but $2100 is a lot for the base model.

Personally I would be going for the 8+ over the 7+ model as it's likely the 8+ will be supported a year longer than the 7+ so you'll get a bit more life out of it.
Thats the logic I look at anyway.

scuwp: with Apple (or anything really) it pays to buy the latest model you can to ensure the longest possible use. Given you still have a 6 one would assume you are not someone that updates regularly so having something supported for a long time would be of value to you, yes? . Why buy something that’s nearly obsolete? The camera in the 8 is amazing.

The problem is that the battery possibly won't last the full life, so it will likely need an expensive repair to replace the battery after a few years. I just wish they had a cheaper way to replace it, rather than using a third party repairer. I tend to buy iphones that are a couple of generations old, because they are still fast, and last a few years due to ios support being 4-5 years now. Usually it is the battery that fails before anything else. Currently got the iphone 6s, which is a big step up from the 6, and still has headphone jack plus 3d touch. I would totally avoid the 7 for multiple reasons (it also isn't much faster than the 6s), and go for the 8 if buying new , although it is quite pricey when you compare it bang for buck with the XR, which is a new gen design and a year newer.