The primary difference is their reporting and printing. Proposals do not have a financial report. They don't have invoices or income. Projects don't have a long section for your contract terms and conditions (at least not one you can see), and they don't print in a format that looks like a document ready for your client to sign.
Other than that, they share the same structure around phases, roles & rates, consultants, and staffing budgets.
This is why we can create a Project from a Proposal with a simple click of a button.