The main difference between these two is the shimmer. Borrowed & Blue is a creme while Sky has a bright blue shimmer. Sky also looks a bit green when compared to B&B. I would say they're different enough to own both.
If you would be happy owning just one of these, go for Borrowed & Blue. The formula is awesome, and it's opaque in just two coats. Sky is tricky to apply and takes 3+ coats to even it out.
Top to bottom: B&B, Sky, B&B
2 coats of B&B and 3 coats of Sky