Chris Coyier from CSS Tricks compares LESS to SASS in the most fair post I've seen. His conclusion:
SASS is better on a whole bunch of different fronts, but if you are already happy in LESS, that's cool, at least you are doing yourself a favor by preprocessing.
Most arguments I've read are just "I like SASS better." His is very technical comparison of the two. It's a really interesting read.
(via David Kaneda)