I wanted to add one note to this discussion because although it's focusing on the camera functionality of the iPhone 6s vs. the 8/X there is one very Japan specific plus to owning the newest Apple iPhones...
Suica.
Japan has it's own contactless payment system called Suica and it's a standard used all over Japan. It goes by many names but Suica, like the others is based off of Felica and can be used in most major cities across Japan. It can be used for trains, some buses, vending machines and Japanese convenience stores (Referred to locally as Konbini) It's a contactless RFID card that can be topped up with cash at train stations and konbini. Suica compatibility was introduced with a Japanese specific model iPhone 7/7s but has recently been expanded to ALL iPhone 8 / X models of any region.
Having this feature integrated to the iPhone is a huge time saver. You can avoid lines by using Apple Pay to recharge your Suica. You don't have to line up to buy individual ride tickets. It's also used to upgrade to a reserved seat when taking commuter trains. You can even set it up to work when the phone is locked. (no need to use biometrics before taping it onto a RFID reader.) It will work regardless, screen off.
Not related to the Camera, but certainly worth considering. I know this is a very specific use case but @OP did mention Japan specifically.
Suica.
Japan has it's own contactless payment system called Suica and it's a standard used all over Japan. It goes by many names but Suica, like the others is based off of Felica and can be used in most major cities across Japan. It can be used for trains, some buses, vending machines and Japanese convenience stores (Referred to locally as Konbini) It's a contactless RFID card that can be topped up with cash at train stations and konbini. Suica compatibility was introduced with a Japanese specific model iPhone 7/7s but has recently been expanded to ALL iPhone 8 / X models of any region.
Having this feature integrated to the iPhone is a huge time saver. You can avoid lines by using Apple Pay to recharge your Suica. You don't have to line up to buy individual ride tickets. It's also used to upgrade to a reserved seat when taking commuter trains. You can even set it up to work when the phone is locked. (no need to use biometrics before taping it onto a RFID reader.) It will work regardless, screen off.
Not related to the Camera, but certainly worth considering. I know this is a very specific use case but @OP did mention Japan specifically.