The difference is clear if you go to Ghost Pro's pricing page[1], which looks just like SquareSpace's or WordPress'[2]. The product is the software, support, and hosting. Ghost is focused on providing value directly to the writers. Substack doesn't have a pricing page and their incentives are much more murky. They want to attract writers and subscribers to the platform, but they don't really care if it is the best experience for either group. If they think of some feature that writers hate, but it increases the number of subscribers or some other metric, Substack is going to implement it.

1. https://ghost.org/pricing/ 2. https://wordpress.com/pricing/