I went to North Uist this week 2 years ago, no wind at all, beautiful beaches but a lot of them are not easily accessed by van. However this week looks great. I think you have to monitor the weather and make the decision a few days before based on forecast. Great people and you do have the feeling whilst being there that you are on an adventure.
Tiree would be my choice next time, all beaches have good access, but again wouldn't make a decision until right up to the week before.