I have done the bus rides and have posted it somewhere here on flyertalk but can't remember which thread. We took the bus from Vina del Mar to Mendoza - it was a pretty nice bus with the seats going all a long way back (i.e. substantial recline though not as much as a UA C seat). This was a day trip and the bus was not more than 35% full. The driver/conductor saw me taking photographs at a few stops as well as along the way - so invited me to sit in the front seat (the one normally occupied by the conductor). This was a truly remarkable experience as I got to sit here while the bus climbed up towards the Cristo Redemptor? tunnel. There were a couple of Andean condors riding the thermals which made it even more enjoyable.
A few miles past the Argentine passport control, and if the day is clear, you will be able to see Aconcagua from the road though it isn't a very long viewing period. If you like scenery, this is a great road to be on. If you are comfortable with driving in Latam countries, this would be a good road to rent a vehicle and actually drive.
On the return we took an overnight bus that leaves Mendoza after 11 pm and gets into SCL around 6 am. We did this about 2 years ago and at that time the bus tickets were between 6-8$ one way. Hope this helps.