Both, and the right choice depends on how much of what you need already exists.
Customizing an existing platform
Faster, cheaper and lower risk. The base is already proven in production. DSP Acquisition provides full source code, so extensive modification is possible without starting over — see DSP License vs DSP Acquisition.
Building from scratch
The right answer when what you need is genuinely novel, or when existing architecture can't accommodate your requirements. More expensive and slower, but unconstrained.
How to decide
Ask what fraction of your requirements an existing platform already meets:
- Most of them → start from the existing platform.
- Some, but the core is different → discuss it; the answer isn't obvious.
- Almost none → build from scratch.
The common mistake is choosing "from scratch" for the wrong reason — because the existing system is unfamiliar, or because building feels cleaner. Six months later you have re-implemented what already existed and still lack the differentiator you actually wanted.
Ask for both options in your proposal. A good scoping conversation compares them honestly rather than defaulting to the larger project.