Buenos Aires to Santiago via Mendoza would be really scenic, but it is 1400 km and we probably do not have time, although it's worth checking out. I wonder if there is a train...?
Sadly, no trains. You can take overnight buses from BsAs to Mendoza (or probably connect from Iguazu to Memdoza with one connection) so you don't lose a day. Lie flat seats, meals, drinks, pretty comfortable. The bus Mendoza to SCL is a very scenic trip, about 8 hours. I traveled all over Argentina and over to SCL that way. I mostly used this company:
http://www.viabariloche.com.ar/index.php
Spring for the first class/ejecutivo seats.