I can't answer much of your question because I don't know but.... Part of Princess's problem is that they often use chartered aircraft to get passengers to the destination and by virtue of that fact passengers comfort is never guaranteed. Buenos Aires is a very long flight and Santiago even longer so I agree with you that I would want some kind of guarantee that it was not going to be a "cattle truck", some charter aircraft are fine but not all by any means. If they are using scheduled airlines for this one then you should have a little more comfort, so ask Princess and then make up your mind. Sorry if that is a bit negative but it is a long way. ... Neil.
I too have suffered the packaged long haul charter flight, but that was 12 years ago & would never go through that again. Since then it's 'cruise only' & book my own flights & transfers with airlines & airports of MY choice. It always works out cheaper by a fair margin if you know how to play the system, not very difficult now we all have internet, the only exception being the last minute, go next week package.
We travelled with Thomas Cook last year and paid for extra leg room going out - we couldn't get it coming back and it was very uncomfortable. Next time we will ask for this when we book the flight, it didn't cost that much and was well worth it if you have got that option.