I came across this Apple job posting at Authentic Jobs for a UI/Visual designer earlier this week. If you apply you get bonus points when you have experience with LESS. No mention of SASS, which to me seems more popular.
We (at Interactive Studios) use LESS. I checked out both LESS and SASS but liked LESS more, so that’s what we went with.
The differences are small, but the main reason I liked LESS more is the simpler syntax for mixins. In SASS you have to define mixins with special syntax, while in LESS you just define a CSS class which you can reuse. Somewhere Apple seems to agree.