OK, I am truly stuck here, and just cannot seem to make a decision. My problem is, I am running two very old (30+ year) furnaces in my home (I live in the second floor of my duplex). Both furnaces are working, and have yet to have a problem since I have lived here. My question is, should I replace them for something more updated and efficient? I only plan on living in the house for 3 more years, and then will either sell or rent it out. I appreciate any feedback and thoughts.

I would suggest replacing both with newer units. Your units should be about 50% efficient. New units range from 85-95%. Depending on the size of the units and the efficiency, the cost will range from $2000 to $5000 each. Installation and warranty should be included. Ask for a parts and labor warranty of at least 5 years. Ten years is also common. Make sure you get at least 3 quotes on both 85% and 95% types. If your heating bills are $400 per heating month (figure 7 months per year), then your savings per month would be $165-$190. The savings would pay for the new furnaces in 4-5 years. If you are going to sell the place, the new furnaces would be a great selling point. This can add several thousand onto the selling price. And with today's market, this will greatly help.
I replaced my old 50% unit with a 85% unit. The pay back in my area was 5 years. The heating guy tried to get me to further upgrade to the 95% (extra $1200). The payback was over 10 years. Since both were better than what I had and money was a bit of an issue then, the choice of the 85% unit was easier - after I did the math.