Why not just make it optional? Save settings permanently (db) or for session only (cookie).
Mobile browser can be detected, it’s really easy so you can have different settings for your mobile.
Cookies are temporary and for session specific setting. It’s easy to use cookies for other crap too but this causes all sorts of annoyances (as mentioned before)
You should really wipe all cookies every time your browser starts/closes and probably run bleachbit once a day to get rid of really nasty and peristalsis cookie like crap that gets stuffed to your HDD by trackers.
To pull yourself out of this blissful ignorance, install Ghostery. You will be mazed how much crap is pushed to your browser.
You like to see sites/trackers who shares information about you? Install Lightbeam. Nice eye opener 
and looks cool too.
It’s time to wake up…