Not every client supports alt text yet, but more are adding it. It's the accepted way to label images on the web, so should be here as well. The more people that use alt tags, the more clients will be likely to add the feature.

Thank you for thinking of accessibility.

I don't understand why you would put image descriptions in the post itself. Anyone who would benefit from them can read them as alt text.

I prefer alt text, and I wish some services, such as Instagram, made the feature more prominent, like on the main postscreen and always on by default, instead of hidden at the very end of additional post settings which most users probably don't have reason to access regularly. Mastodon has this feature turned on by default for every user, and I am impressed at how many people use it.

I prefer the former rather than the latter, as using captions within the post means that the person posting has less characters to work with in the first place.