It depends on your project needs. MERN Stack is a specific type of full stack focused on JavaScript technologies—MongoDB, Express.js, React, and Node.js. It's perfect for building modern, fast, single-page applications.

Full Stack is broader and includes various technology combinations (like LAMP, MEAN, Django + React, etc.), giving you more flexibility based on your requirements.

If you want flexibility and a range of tech options, go with a full stack development company. If you’re building a modern web app with JavaScript across the board, MERN is a smart and efficient choice.