I am planning to travel a 'circular' route around Argentina in 3 weeks.
My rough idea for the route is Buenos Aires -> Puerto Madryn -> El Calefate -> Mendoza -> Cordoba -> Salta -> Iguazu -> Buenos Aires, but this is not exhaustive and will probably change.
When am I better off taking an aeroplane because either the scenery is boring or the distances are crazy?
None of it is particularly scenic. I've bussed most of those routes, and for the most part - especially on the plains, it's fairly standard.
The BA to Puerto Madryn was like 20 hours. It was striaght and smooth and a very easy bus ride. Most of it is dull though, flat and ordinary, but you can bus overnight and it's not too bad.
I'd suggest Puerto Madryn to El Calafate by bus as it's still initially flat, but then as you get closer you get more into the foothills and mountains of Patagonia. It's a long way down though, and takes several days to get back to Mendoza, so perhaps at that point fly.
If, however, you would consider Santiago, or can get a cheap flight to Santiago, the 8 hour bus ride between Mendoza and Santiago in Chile is spectacular - seriously spectacular!
Between Mendoza and Cordoba it'd be easy and flattish, with more hilly stuff closer to Salta.
I flew from BA to Iguazu, but I imagine it'd be quite nice to see the landscape as it transitions to rainforest.
In the end it'll come down to time, comfort and money. The buses in Argentina are supremely comfortable - often for just a few dollars more you can get a lie-down bed in full-cama / cama-suite class, instead of the tilty-seat semi-cama.
Also note that all Aerolineas Argentina flights hub-and-spoke through BA, so from say, El Calafate to Mendoza, you'll be flying back to BA and then out again to Mendoza. Costs add up. I flew my first couple of legs, then discovered I really enjoyed the bus trips and had the time for them, so continued to bus from then on, all the way up to Colombia.
Most of the locations you want to visit are very distant from each other, perhaps you could take a Bus from Mendoza to Cordoba (I think it's a trip of 500km). One disadvantage when traveling inside Argentina is that most cities are connected through Buenos Aires.
