RFPIO is purpose-built for RFP response and sometimes a bit too cumbersome for security specific needs. Though their answer bank and automation features are nice, their business is built on helping you answer security questionnaires (so the more you get the better).
Conveyor is a 2-in-1 solution - offering both secure document sharing and a knowledge base of answers that you can expose to customers and use internally to answer questionnaires. By using both of these features, you can reduce the number of questionnaires you receive overall.