You may think it is an easy question but I assure you that it is not.
Are you going to take full responsibility that the HVAC unit is correctly sized? Are there multiple rooms in this construction? Has a room-by-room heatloss/heatgain calculation been made and the ducts sized accordingly? What kind of terminal units (air registers) are you specifying? What R factor do you want on the flexible duct? What is the cost of various diameters of flexible duct ?
Are you going to use a union sheet-metal shop to fabricate the transitions from the HVAC unit to the round duct? Are you planning on a "star" distribution or do you want a plenum duct with branches? Is the duct installer going to be a skilled worker with experience or a handyman that you found in an envelope of Val Pak coupons?