Both stacks can ship production SaaS. The choice depends on team structure, deployment model, and how much you value unified routing.
MERN strengths
Clear separation between API and client. Familiar for teams with dedicated backend engineers. Flexible hosting for the API layer.
Next.js strengths
Unified codebase, Server Components, built-in routing, and excellent Vercel deployment. Strong for solo developers and small teams moving fast.
My recommendation
For new SaaS products in 2025, I default to Next.js + TypeScript + Prisma unless you have a specific reason to split frontend and backend across separate repos.
Speed of iteration wins early-stage products.