L90 is quicker, the walk from the Mall is short. L85 gets very close but stops at more stops. If you get the train, go to Wynyard because more buses go from there than from Central (only L90, L88 and express buses go from Central to Northern Beaches). 131500.com has a good trip planner (which includes buses and trains) and all bus timetables, cityrail.info has all train timetables. All buses mentioned run every day.
That being said, it'd be a LOT quicker and a lot easier if you drove. Public transport is notoriously unreliable in this area I'm afraid (I speak from much experience).