For any upgrade, I consider whether I’m likely to benefit from the new features in a significant way. E.g. the new Face ID on the iPhone X is cool, but it’s more of a “nice to have” than a feature rather than something that will significantly enhance my day-to-day use of the phone. The wireless charging is a lot more interesting to me, especially with the AirPower charging mat that can charge up to three devices at once. That’s available on the iPhone 8 as well, so I might end up getting that. I’m currently using a damaged iPhone 5C, so any of the current iPhone lineup will be an significant upgrade.
Normally iOS games work on older devices too, so it seems unlikely that Sky will require one of the new iPhone models to run. But it won’t hurt to have that extra power either, especially as the game is being developed with the 4K Apple TV in mind.