The verdict
If you run a small B2B sales team with a CRM and send contracts as well as proposals, I'd pick PandaDoc. There is a free plan to try, and once the templates are built, preparation time drops.
If you are a freelancer or a small agency who wants tidy proposals, tracking and an invoice-ready workflow at a low cost, I'd pick Prospero. The single plan makes the choice easy.
The risk with PandaDoc is setup effort and per-seat cost as you grow. The risk with Prospero is thin CRM links and no tier with approvals to grow into. Match the tool to the stage you are at, not the one you hope for.