San Juan is nice....it's been some years since I've been there but it's not very large from what I remember. Salta is very nice....it's where my wife and I honeymooned for 3 days....I like Salta and "Las Empanadas Salteñas" with the spicy chimi style "aji" dipping sauce....delicious. They sell them on the streets, they are great... if you go try some. Salta is fairly mild, kinda tropical from what I remember as it's not as high in eleveation. But if you visit some of the outlying towns, especially up towards the "alto-plano" like "El Valle Calchequies" where their can be some drastic changes in elevation....it's quite different.
When we went in Salta it was July, like a balmy 22-23 degrees C. About 2 hours later after we arrived in "Payogasta" which is a little adobe andean town in near Cachi" in the alto-plano it was like -3 LOL...but we were up at like...I dunno 8 thousand feet elevation up there from Salta.....it was freezing!! We wound our way up in the bus and up and up..just winding along these dusty mountain roads. On one side of the road a sheer vertical cliff or heavy incline, on the other side, a steep escarpment like 2 or 3 thousand feet down looked like...I mean WAY down and straight down!! Salta is very Andean and actually that whole section of Argentina smacks more of Chile or Bolivia then Argentina to me...it's very different in the north....MUCH more laid back, and less Lunfardo LOL..