Should I use a Micro:bit or Arduino or something else?
Honestly, it really doesn't matter.
Arduino is basically an Atmel AVR with a snazzy board and a lot of beginner/education friendly peripherals and a development environment.
Microbit appears to be the same thing, but with an ARM Cortex M0, and a less mature community.
In both cases, if you were making a custom PCB for commercialization, you'd create a new board around the chip you're using, the AVR or a cortex M0, rather than basing off the arduino or microbit designs. You'd probably also ditch the beginner friendly dev environments.
So, it ultimately boils down to which chip do you want to depend on. ARM chips have a lot of capability for the price, and you can get chips from many different manufacturers, but their tooling is considerably less friendly than AVRs.