Re: Which development environment to use for ATSAMC21
Only you can make that call.
You will have to compare them, read/watch some few tutorials to see which one you might be conformable with.
Check the features, the supported libraries (especially related to what you want to develop), etc.
In the future I'm not sure whether Microchip will continue to support the software tools it inherited form Atmel or will slowly move them into Microchip existing tools. I have seen they are now supporting more and more previous Atmel devices in MPLAB Code Configurator, Harmony and their new hardware tools like Snap, Pickit4, etc.
My guess is this is where most of development will occur in the future, I am not sure if the same is happening with the previous previous Atmel tools.
So if all these platforms meet your requirements, then I would choose the one which is likely to be more developed in the future