Big Sur beta on external disk vs internal disk?
In general, I would recommend installing it on an external SSD disk. First, you won't have to worry about possible disk issues with your regular MacBook. Second, it is easier to erase and manage with Disk Utility.
Just as long as you know how to install correctly, I would go for that option.
Edit: While setting up a separate APFS works well in many ways, such as dynamically adjusting the size, the slight risk is still present in case things go wrong.