Just depends on how the present ducts are run. In my house they cheaped out, ran very bad quality ducts and laid them out improperly. I could never keep the back of the house cool or warm as there was just not enough airflow. I had the furnace replaced and they strongly suggested I get new ducts. I didn't and the new ho efficiency system didn't do any better than the old one as far as keeping the temps even.

A few years later I few of them started to break down and I had to wrap them in plastic and aluminum tape. I eventually called them back and had them run new ducts. WOW! What a difference. The ducts were run much better and made attic access much easier. The entire house stays within a few degrees from room to room. The new ducts are far quieter too.

It seems odd that 2 companies say the same. Maybe try and get a 4th estimate and see what they say.