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 is not a Linux distribution as the project's motto states "It's not an embedded Linux Distribution, It creates a custom one for you." It means that you can use Yocto to create a custom Linux distro for your device. Yocto uses a modular software approach that lets you select only the specific libraries and tools required for your system.
This makes Yocto an ideal framework to build an embedded system for many types of systems. Debian, like other Desktop Linux distributions, 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 larger distro with less flexibility and it is harder to customize and maintain, but sometimes, it is faster to bring-up and add to it precompiled packages over the internet using its strong package manager.
On the other hand, 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 customize and fine-tune any aspect of the distribution quite easily. I would say that Debian is good for fast prototyping or proof-of-concept but Yocto would be my choice for the production-ready product.