iPhones are built with a closed controlled system which do not supprt OTG in the real sense. Some tech giants have developed technology to bring surrogate storage accessible by an app. But that isn't as seamless as OTG.
Though Sandisk has come with a extra storage drive for iPhones with are plug and play devices
Apple IPhone 7 Plus vs Apple IPhone 7S Plus
Both the phones have their pros and cons,Apple iPhone 7 Plus would be better in term of all specs and performance:-
Advantages of ,Apple iPhone 7 Plus
1.Has Dual Camera:-Dual camera is useful in enhancing certain effects like blur, zoom and wide angle photos.
2.Has Fingerprint Sensor:-Fingerprint sensor support allows you to quickly unlock your device using your fingerprint.
3.Supports NFC:-NFC enables wireless payments through your device. You can also connect two NFC enabled devices just by touching them with each other.
Advantages of Apple iPhone 7s Plus
1.Faster Download Speed:-Apple iPhone 7 Plus. Faster Download Speed means stream videos at real time.
Verdict - In my opinion, Apple iPhone 7 Plus is better overall performance and price ,
You can also compare these devices on 91mobiles