[Q&A] CM 12.1 nightlies vs CM 12.1 OS? (+ other)

Hi guys, i've searched an answer to this but nothing so here I am.

What are the main differences from CyanogenMod 12.1 Nightlies and CyanogenOS 12.1?

The OS is build especially for the One plus One and so?? The battery drain is different? The desing??

Do you recommend one or another?

And for finish this, why I would install a rom on my smartphone built on Cyanogen? Why I would change the kernel?

Thanks