hiring different developers to build different parts and then you join them together is risky in a sense that the code is not uniform across all parts, therefor

hiring different developers to build different parts and then you join them together is risky in a sense that the code is not uniform across all parts, therefore it can be one part is efficient, one part is not. hiring one developer to build them all is good in a sense because it's consistent across the board. the bad point is that it's one developer only, it takes time (vs multiple developers). if you decide to go with multiple devs, choose a tech that is mainstream such as Zend or CodeIgniter vs Ruby (while Ruby is getting popularity, but its pool of devs is not as many as Zend).