Hi, possibly too late now with this reply as you may already be there.I've not been to Lefkas but have been to Santorini.I stayed there for a fortnight which was a bit too long for me but it is worth a visit.I originally went on a day trip from Paros and the approach to Santorini gives you absolutely breath taking views.You will land at the main town of Santorini and that really is the best part of the Island and you should enjoy the day there.
Hope you enjoy your holiday.I love the Greek Islands