Substack may be the It Girl of the moment but ultimately mailing lists and individual subscriptions are hopeless dead ends - the recent changes they've announced (facilitating cross-list recommendations) suggest they realise that.
Medium can be improved for sure:
https://medium.com/youve-been-informed/making-medium-magnificent-906211fd6044
but Medium's one-subscription-supports-all model is far more sustainable in the long term.
I fear some writers will be seduced by 'cool girl' popularity (as is suggested by some of the replies here) into making a decision that will be self-defeating.