Perhaps the most important decision that you need to take when starting a new embedded system project is which embedded distribution will best fit your project requirements.
Yocto is probably one of the first candidates, it allows you to create a fully customized Linux distribution for your device. Yocto uses a modular software approach that lets you select only the specific libraries and tools required for your system.
The Yocto build system is well constructed and documented, it may take some time to learn how to use it, but once you control it you can create the perfect Linux system for your needs.
On the other side, Debian is a full Linux distribution for general-purpose computing.
Developers describe Debian as "The Universal Operating System”. It was designed to run mostly on Intel-based systems with a mouse, keyboard, and monitor. Yet, Debian can also run on many embedded architectures.
Debian is a large Linux distribution with less flexibility, it is harder to customize and maintain. But sometimes it can be faster to bring-up Debian on embedded devices and use its package manager to install precompiled packages over the internet.
I would say that Debian is a good choice for fast prototyping or proof-of-concept, but Yocto would be my choice for production-ready products.