I am trying to decide between a Rheem high efficiency furnace and an American Standard. Which furnace is better of the two and which one of the two is most economical ?

You will get any number of contrasting opinions. Ask around to friends and neighbors. Talk to unaffiliated service companis As far as efficiencies, the AFUE rating tells all.They have to meet certain standards. Minimum is 78%. 22% of the heat is wasted. Mine is 92%. The real cost is what you pay for the gas and how your system is designed and installed. If you have a 50 year old house, you have leaky metal ductwork. It should be sealed. Use a setback thermostat. Be reasonable about temperatures. Many things to consider before saying brand A over brand B.