I'll agree with all of the previous comments. The bus is the best. Though it is still worthwhile to add some independence if you choose to rent for a portion of your your trip. Try not to drive at night; lack of headlights, potholes, animals, and trucks all make it difficult. From Mendoza, I have enjoyed drives north through San Juan and then La Rioja provinces, at least to Cafayate. The highway farther north to Salta is not that great. Parque Nacional Talampaya, and Valle de Luna are worthwhile. Possible nice start from Mendoza is Uspallata into Barreal for the much longer scenic way into San Juan. Salta 2-3 day trips into the high canyons, or old desert towns north of Jujuy are nice. 2-3 day trips south from Mendoza into San Rafael areas. Seriously though, avoid the night driving.

Automendoza may have surcharge free one way car rentals since they have offices in San Juan. You can rent a car to Chile, but the companies only have insurance papers for a limited number of their cars to cross the border. There are 5-6 companies all on the same street as the Sheraton Mendoza. Hertz, etc, and some local companies.