When comparing F.lux vs Little Snitch, the Slant community recommends F.lux for most people. In the question“What are the best Mac OSX apps for someone that's new to Apple?” F.lux is ranked 5th while Little Snitch is ranked 7th. The most important reason people chose F.lux is:
Helps provide better sleep for more energy during the day.
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Pros
Pro Can help sleep better
Helps provide better sleep for more energy during the day.
Pro Eases eye strain
Decreases color temperature at night, changing the normally fluorescent blue of an LCD monitor to a soft orange. Much easier on the eyes, especially in a low-light environment.
Pro Subtle transition
The transition from daytime to nighttime color modes takes an hour by default and will detect your current timezone, meaning that it will happen in the background at sunset, often without you noticing.
Pro Protected even at login screen
Little Snitch rules work even at login screen. Even when receiving attempted connection alerts, once logged in LS has stopped and requested permission for them.
Pro Comparatively easy to use
Easier to use than traditional firewall software, but more powerful due to design and workflow.
Pro Protects your privacy
Protects your privacy and helps to prevent unwanted connections.
Cons
Con Takes a while to get used to
After installing F.lux, the screen will look a lot more yellow. This takes about a week to fully get used to. You can ease into it by setting the color temps higher than the recommended defaults.