I don't know what Substack's whole deal is, so I'll have to look more into it before I pass judgement. I don't think Medium's payment calculation is that complicated; it's based on Member reading time (how long paid Medium subscribers spend on your stories), plus the time spent by anyone who becomes a paid member within 30 days of reading your work, plus referrals. It's paid out via Stripe which is easy enough to set up. If cross-posting isn't against the terms of service of either platform, I see no reason why not to do it, but I don't know much about Substack so let me know how that goes!