Using bufio.NewReader will behind the scenes call bufio.NewReaderSize which hash this documentation:
// NewReaderSize returns a new Reader whose buffer has at least the specified
// size. If the argument io.Reader is already a Reader with large enough
// size, it returns the underlying Reader.
So if there is any difference depends on the io.Reader returned by os.Open. If the buffer of it is ‘large enought’, there is no difference. The default buffer size is 4096.