That's a difficult one. I love Orkney, been there several times, lots to see and do. They are a much bigger group of island more agricultural and you'd need to hire a car once there to get the best out of them.
I have been to Barra once - read all about it
http://www.flyertalk.com/forum/trip-...sion-long.html It is a small island, one small village as the centre of habitation (stay in the Castlebay Hotel,not the Isle of Barra!). But it is remote and quiet and peaceful. If you are happy with that for two days, then go for it. The beach landing is very special as to be honest so is the flight.