Monolithic source code repositories (repos) are used by several large tech companies, but little is known about their advantages or disadvantages compared to multiple per-project repos. This paper investigates the relative tradeoffs by utilizing a mixed-methods approach. Our primary contribution is a survey of engineers who have experience with both monolithic repos and multiple, per-project repos. This paper also backs up the claims made by these engineers with a large-scale analysis of developer tool logs. Our study finds that the visibility of the codebase is a significant advantage of a monolithic repo: it enables engineers to discover APIs to reuse, find examples for using an API, and automatically have dependent code updated as an API migrates to a new version. Engineers also appreciate the centralization of dependency management in the repo. In contrast, multiple-repository (multi-repo) systems afford engineers more flexibility to select their own toolchains and provide significant access control and stability benefits. In both cases, the related tooling is also a significant factor; engineers favor particular tools and are drawn to repo management systems that support their desired toolchain.