What are the differences between PowerShot firmware and EOS firmware?

[lumenite]

Hello, I just joined this wonderful forum, thinking that ML looks very interesting.
I have two EOS Ms: the original M and M5.
ML site says that M's firmware is based on that of EOS DSLR, while M5 on PowerShot.
What differences between them?
I think the answer must be somewhere in this forum, but I confess my inability to search it out.

Their appearances in menu display are clearly different from each other. That's it?
ML site seems to say that M50's firmware is also back to EOS. But its menu looks like M5, rather than M.
One thing that I am sorry about M5 is that IS operation of EF/EFS lenses with M5 is different from M's working. Is it because their firmware base is different? Then, is M50 okay in this regard?

What is the main issue of the difference?
What points can be noticed from the viewpoint of the end user?

Thank you.