Luskentyre Beach on Harris is my favourite beach in the world. I was lucky to visit it in weather like this, and there was barely a soul on it.
Barra is lovely, as is Eriskay beside it, but limited in what you can do. I got bored after about a day and a half.
If you only have a few days, I'd base yourself on Harris, and then you have the option to tour about there and Lewis.
This is where I stayed. Breakfasts were superb...(disclaimer. It was over 10 years ago)