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 6s 32GB vs Apple IPhone 7 Plus
Advantages of Apple iPhone 7 Plus
1.Quad Core CPU:-More cores result in better parallelism, meaning more tasks can be processed in parallel without slowing down the User Interface.
2.Faster CPU:-Faster CPU means more smooth experience.
3.Bigger Screen:- Reading, browsing internet and watching movies is more pleasing experience on a bigger screen.
4.Has Dual Camera:-Dual camera is useful in enhancing certain effects like blur, zoom and wide angle photos.
5.Bigger Battery:-It can generally last more time.
Advantages of Apple iPhone 6s (32GB)
1.Light Weight:- Light weight devices are easier to hold .
2.3.5mm Headphone Jack:-Having 3.5mm Jack enables you to connect to large number of other devices (such as speakers) having 3.5mm Port.
Verdict :-In overall performance and price you should go for Apple iPhone 7 Plus or else you want light weight handset go for Apple iPhone 6s (32GB)
iPhones are built with a closed controlled system which does not support 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. Currently No. In the future: Maybe but highly unlikely