"1MW" is the proper case.

"1mw" would at least convey you are consistently irreverent of the proper case, or using an impractical onscreen keyboard, or in a rush to type this post. (We would understand.)

"1mW" on the other hand makes you look uneducated. Inconsistently inconsistent. You care about the case sometimes, but not always. You have no excuse to write it this way.