It looks like the 32GB iPhone 7 has significantly slower write speeds, based on and (between 3x and 8x slower in some benchmark apps although the read speeds are about the same). The main area where you would notice the difference is in transferring large files, such as movies.
Not a dealbreaker but certainly something to bear in mind.
The need for (write) speed
That discrepancy might be due to Apple using a different type of flash memory inside the 32GB iPhones, but it's also an established fact that smaller solid state drives simply because they use fewer parallel channels at once.
However, the size of the discrepancy seems unusual, even accounting for the way solid state memory works. As yet Apple hasn't responded to the claims - and so far no one's run the same tests on a either.
In most of your iPhone operations you're not going to notice the difference, so don't think the 32GB iPhone 7 is broken or defective; but if you do spend a lot of time transferring large files to and from your device, you might want to think about getting a bigger model.