It's about a 5 hour drive from Boston to Acadia without traffic; if you land during rush hour, you can probably add an hour to that, and maybe another 30 minutes due to it being the Thursday before Memorial Day, as lots of Bostonians are driving to Maine that weekend. It's much shorter from Portland, but you said you wouldn't rent a car from PWM, so I'm sure you've looked into how long it would take by whatever transportation method you would take. My personal preference would be to take the connection, as I don't like long drives after a cross country flight, but it does depend on the price difference.