David,
“You are better off” is used when you already have a negative notion about something and you try to avoid it.
The traffic is bad to the airport. I think you are better off to take the Metro Rail to the airport. You are suggesting an alternative which has no threatening tone.
“You’d better tell your dad about the accident before he finds out”. This hints resultant consequences.
They may seem interchangeable to you. But they are not quiet the same in tone.
<<"Because of the increased security, I think you are better off arriving at the airport early so that you won’t miss your flight?>>
"By the way, “arriving” is the state of getting there which is not the case. So I would still use “to arrive”. Just my take.