Filipe,
As informações seguintes foram extraídas do site abaixo indicado. Acho que só pelo visual, essa sua trip (Santiago-Mendonza) já deve valer a pena. Por outro lado, vindo de Buenos Aires fica mais cômodo, pois o ônibus é semi-leito e vai direto pra Las Leñas.
http://www.laslenasski.com/view.asp?p=36
You can also take the bus from Santiago, Chile with departures from 08:30 AM-1:30 PM to Mendoza and then the Tac bus from Mendoza to Las Lenas the following day at 6AM (Must spend 1 night in Las Lenas). Going overland from Santiago is on of the biggest junk shows in South America and see Santiago section for more info. The bus from Buenos Aries is a special night bus with comfortable big seats and dinner is served on the bus and there is usually a movie and leaves daily at 8PM.