If you were in Mendoza, and wanted to then travel somewhere for approx. 2 days (I know, short) before heading back to NYC, in May, where would you go: Salta or Santiago? Salta seems to be getting a fair amount of support on here, but my friend from NYC who spends time in Mendoza every month recommended Santiago.

Simply for ease and expense, Salta would make more sense. I always think that if you only have 2 days you are better sticking to the country you are in rather than try to squeeze in another one. It does depend on how you like to travel, as some people do like a fast pace, but I would recommend sticking to Argentina especially as your international flight is out of there.

I've spent time recently in both and although both are good, Santiago is still just a big city. Recommend Salta unreservedly. Do at least one full day trip driving either north or southwest.

another vote for Salta

..and another for Salta ( with maybe a drive out to Cafayate if you have time

Definitelly, Salta is the best option!!!
Enjoy the NW!
FLintstones...

Haven't been to Salta, but if I were headed west it would be to Valparaiso, not Santiago. I might do that just for the drive over the Andes.