24 x 25 will be slightly cheaper than 20 x 30 in terms up framing, sheathing, siding, and insulation materials.
24 x 25 x 8 = 784 sq ft
20 x 30 x 8 = 800 sq ft = an extra 2 studs (assuming 16" centers), partial piece of sheathing, 2 pieces of siding (assuming hardi plank siding), and an extra...