Count the fee on member payments as part of the price
Circle, Mighty Networks, Skool and Patreon all keep a share of what members pay, and on cheaper plans that share can cost more than the subscription. Skool's Hobby costs $9 a month and takes 10% + 30 cents per transaction, against $99 and 2.9% + 30 cents on Pro. Mighty Networks keeps 5% on Explore, at $35 a month, and 2% on Launch, at $95 a month or $79 billed yearly. Circle keeps 2% on Professional, $89 a month on annual billing. Both add Stripe's fee on top. Patreon has no monthly fee and keeps 10%, plus processing, so on $2,000 a month it takes $200 where Circle's 2% is $40. Compare totals at the revenue you expect, not plan prices.
Check the apps and email options
Members need a way back in. Circle has web, desktop, iOS and Android apps from Professional up, and Mighty Networks includes its iOS and Android apps on every plan. Discourse has iOS and Android apps with push notifications for hosted communities, and members can reply by email. Skool sends in-app and email notifications, including digests of popular posts, and Patreon has iOS and Android apps. An app under your own brand needs Circle Plus or Mighty Pro, both priced by quote.
Decide whether you need public or private
Discourse is built for public discussion: public sites serve search engine crawlers a server-rendered layout, with robots.txt and indexing controls, which suits a support forum or an open-source project. Circle and Mighty Networks can be public too. A Circle owner chooses which spaces logged-out visitors see and manages indexing in the SEO settings, and Mighty Networks gives control over search indexing from the Scale plan, though not during a free trial.
Check what you can take with you
Moving later depends on what each platform exports. A Discourse backup moves a community between self-hosting and Discourse's hosting, and its Business plan includes a standard migration from Vanilla, phpBB, vBulletin, Slack or Drupal. Circle exports members as CSV files and lists payments migration on Professional and a standard migration service on Business. Mighty Networks exports member data as an Excel file from Launch and offers migration services on Growth, and Patreon lets creators export their email lists. Before committing, check whether posts and active subscriptions can move too.