The verdict
If you are a freelancer or a small agency and you write most proposals yourself, I'd pick Prospero. It has one published plan, with no tiers to untangle.
If you are a sales leader with several reps and want approved content, right prices and approvals before anything goes out, I'd pick Proposify. Prospero is a sensible start, and Proposify is the step up when a team arrives. There is no single winner. Check the details first. Proposify has send limits and per-user pricing. Prospero's CRM links are thinner, and its price also scales with the team.