We went to Argentina last November and wanted to do Puerto Montt (Chile) to Bariloche (Argentina) or vice-versa. It seems to be a great trip from what we found on the Internet. It was also featured on one of the travel programmes on BBC and looked wonderful there (it involves a series of bus and boat trips).
However, we didn't do it because it would have involved a one-way international flight which would have been very expensive. All our other flights were domestic ones within Argentina. Maybe you should check this out.
I don't know about Santiago to Mendoza but that could be an equally exciting adventure.
As part of our holiday we went on a day trip from El Calafate in Argentina to the Torres del Paine national park in Chile. That was travelling and crossing international borders like it used to be years ago in Europe!
Last year's visit to Argentina was our second; the first was awesome and, as the country is so big and there was so much left to see, we decided to go again. There are still areas we haven't been to (including Mendoza) so.... watch this space.