If you have a credit card from Citibank Argentina, you can always get a 10% discount at HiperRodo (Cabildo in Belgrano) or Falabella, and you can pay in 12 installments without interest.
Other banks have special deals at Coto, Jumbo, and Carrefour, whose big stores carry large appliances. Most stores will discount the price by 3 - 5 % for cash. Small neighborhood stores may do a little more, but few have much stock.
Otherwise, in my experience, there's little price competition among appliance dealers. If you can find what you want in one store, the price is likely to be within a few pesos of the prices in any other store.
Rodo usually has good selection and stock. Alternatives are Garbarino, Fravega, etc.
I've heard that Coto has an outlet store out by the General Paz that offers substantial discounts on slightly-damaged goods and discontinued items. I can't find an address, but you might call their central number and ask.