Short answer: It's complicated - but trust B4UFly above all other sites, because it's tied into the FAA (and developed by them)
Long Answer: It's super complicated. But essentially you need to trust B4UFly because it has more accurate data and it's in real time. For instance, right now if you want to fly near Philly or in the burbs - you can't. There is a Temporary Flight Restriction (TRF) in place for a VIP movement. So it's always good to check that app and see whats going on. You can even use the planner to plan ahead and see if there are any restrictions for the date and time your flying.
Now if you look at the FAA guidelines they vaguely state "I will not fly near aircraft, especially near airports". Doesn't really give much information or miles. But using the app you know that within five miles of an airport you'll get a warning. Basically if you want to operate within that area, you are responsible for notifying the airport of your location. The app can actually get the data by asking you about height you will fly, time, and something else....can't remember. I'm assuming it sends this data somewhere....but if your going to operate next to an active airport - let them know. I have my UHF radio with me so I can monitor tower frequencies and know when people are taking off and landing.
But to make things more complex, you also have to know that if you fly in Class B or C airspace you need to get ATC permission. Most airports the Class B or C airspace doesn't start at the surface, so staying under 400 should be ok...but again - you need to be aware.
So in short...use B4UFly/