The verdict
If you are a freelancer or a small agency sending proposals to one client contact, I'd pick Prospero. It does the job at a low cost and with a single published plan, so there is nothing to untangle.
If you sell to buying committees and run the late stage of the deal from your CRM, I'd pick GetAccept. You get engagement by stakeholder and a shared plan for the buyer.
There is no winner for everyone. Most readers will sit clearly at one end. If you are in the middle, with a small team and a few multi-person deals, test GetAccept's trial and Prospero's side by side before deciding. Mind that GetAccept's CRM integration needs the Professional plan.