Las Lenas is around 7 hours away by bus from Mendoza and Penitentes around 3. Las Lenas is a lot bigger with modern skilifts and a snowboard park with rails, ramps and jumps. It is quite a drive from Mendoza so you would have to stay there for at least a couple a days to take full advantage of the powder.
Penitentes is closer to Mendoza and a lot smaller, and can be done in one day.
Several agencies in Mendoza, especially in Avenida Las Heras specialize in organizing trips to both places and rent equipment.
Going to Las Leñas, even if you are not so kind skiing (like me lol), its def, a must see.
In Las Leñas you will find a great vibe very good slopes, a very interesting social life and an absolutely "no worries" policy all arround.
This year i was there for 4 weeks asisting with Argentina Alive to a company that join us to organize this trip, was an great experience and met lots of extraordinary people from all over the world.