I have an iPhone 6S 64GB, and it works well for what I need. However, I've had it for awhile now, and while the battery is still fine, I am starting to wonder if it's worth upgrading. IMPORTANT: I would like to buy used, and not new, to save $, so that probably means something older than X, maybe a 7 or the newer 8. In your opinion, is it worth upgrading to one of the 7 series or 8 series? Is it a big jump over the 6S? Thank you!