Both ways of making slideshows and sharing them are good. To summarize, since you are intending to share your slideshow with non-Mac users, I would recommend iMovie with its more universally compatible Mp4 format, versus the Photos app's .m4v format that is uniquely Apple's. Also, you get more editing options with iMovie, and, particularly, it is much easier to edit audio with iMovie.
Discussion:
iMovie offers more features and editing options than you would get with a Photos app slide show. It is much easier to edit audio with iMovie versus the Photos app. It also shares out in the Mp4 format that is more universally compatible across platforms than is the .m4v format of the Photos app slideshow.
With the Photos app you would first share the slideshow as a video to your desktop. That would give you an .m4v file rather than the Mp4 file that you would get with iMovie. The format .m4v is uniquely an Apple format that would play on all Apple devices and software, but may not be as universally compatible with Windows and other platforms. So to ensure maximum compatibility you might want to convert the .m4v format to Mp4.
-- Rich