First of all, the size plays an important role. A furnace which is smaller in size will never give your home the desirable amount of comfort. To tackle this problem most of the times homeowners end up buying furnaces which are too big in size. An oversized furnace is also not suitable because it will cycle on and off more frequently, thus leading to more wear and tear of the machine. Hence, a correctly sized furnace is of utmost importance. To achieve that you will have to get in touch with an experienced HVAC contractor who will perform accurate load calculation and suggest you the rightly sized furnace.
The efficiency also impacts the performance and comfort level of a gas furnace. For example, a gas furnace with higher AFUE rating is more efficient and gives more comfort than the one with a lower AFUE rating. At the same time, gas furnaces with higher AFUE rating might be costly initially but in the long run the amount you save on monthly utility bills will make your initial investment worthwhile.
Thus, when you are buying a gas furnace keep a careful eye on the size and the efficiency rating of the system to maximum gains.