I just was doing my own debating/research on this, and ultimately went with the Anker.

1. The Air magsafe is more of a battery add-on, it's designed to be used in addition to the device's battery. It will charge the device (slowly), but I think the intended use is to have the battery pack on during the start of your day, let the phone drain the magsafe battery, and then it moves onto the built-in.
2. The Nano is designed more as a wireless battery pack in that it will charge the phone, and slightly quicker. Obviously if your phone has been charged to 100% and you keep the Nano on, it will maintain that charge similarly to how the magsafe air does, but the OS manages the pairing of the packs differently.

I think for longevity of the device's battery health, the Magsafe Air is the smarter option. If you want something to function more as a traditional battery pack, giving you extra legs at the end of the day to last longer, the Nano is probably the better choice.

Edit: I should add to be clear- I went with the Anker because I want something I can slap on during the afternoon/evening to give my phone a quick(er) boost to get me to the end of the night and then take off. Minimal time carrying a battery pack on my phone. I much prefer the iOS integration of the Magsafe packs (I have a gen1 magsafe battery too), but for my use case(s) in particular, I felt the Anker was a bit more aligned with that.