I have Rosen and Piero (coral and high class) colchones (mattresses). All are innerspring (resortes). I didn't buy pillowtop because I thought it might wear out faster. King Koil is very good, too.
The name brand stores often have 20% off sales, usually with a bank credit card.
You can make price comparrisons here:
http://listado.mercadolibre.com.ar/colchon-1.40-1.90
This is a list of new mattresses. There are lots of used ones for sale as well. I bought two used mattress and box spring (somier) sets on mercado libre two years ago. I paid $450 for Piero High Class 1.40 x 1.90 set and about $800 for a Rosen set 180 x 2.00, including the metal support. I'm happy with both of them. They were in perfect condition with no stains. The Rosen was only a few months old (I saw the invoice) and was being sold because it wasn't firm enough. It was extra cheap because it was offered in an auction (subasta) and not at a fixed price. The Piero High Class had been in a guest room and was like new. I still sleep on the Piero Coral 1.8 x 1.9 that I bought new in 2006 for $999 pesos (mattress only) at the big Piero store on Pueyrredon (or Jujuy) at Belgrano. I bought double box springs (90 x 1.9) for it on Mercado libre in 2010 for about $500 pesos.
I know some might not want to sleep in a "used" bed, but I wonder if they give it any thought when they stay in a hotel.