Request for Proposals (RFPs) and grant applications are both formal documents used to solicit proposals or applications from external parties. While they share some similarities, they serve different purposes and are used in distinct contexts.

Similarities:

1. Both RFPs and grant applications involve a competitive process where multiple parties submit proposals or applications to secure funding or win a contract.

2. Both require applicants to provide detailed information about their organization, their proposed solution or project, budget, timeline, and expected outcomes.

3. Both often involve a structured evaluation process, where the issuing organization assesses submissions based on predefined criteria and selects the most suitable proposal.

Differences:

1. Purpose: RFPs are typically issued by organizations, such as businesses or government agencies, seeking specific products, services, or solutions from external providers. Grant applications, on the other hand, are used by funding organizations, such as foundations or government agencies, to award financial support to non-profit organizations, educational institutions, or individuals for research, projects, or programs that align with the funder's mission and goals.

2. Context: RFPs are more common in the context of procurement and contracting, while grant applications are more prevalent in the non-profit, research, and academic sectors.

3. Financial aspect: RFPs usually involve a commercial transaction, where the selected provider is paid for their products or services. In contrast, grants provide financial support to recipients without the expectation of direct commercial return or repayment, although there may be reporting and accountability requirements tied to the grant.

4. Structure: RFPs generally have a more rigid structure, outlining specific requirements and expectations, whereas grant applications can be more flexible, allowing for innovation and creativity in the proposed project or program.

As for popularity, it depends on the context and sector. RFPs are more prevalent in the business world, where organizations procure goods and services from external providers. Grant applications are more common in non-profit, research, and academic settings, where funding is required for projects or programs. The popularity of each varies depending on the specific industry, economic climate, and government policies.