Depends on the slope of the outfall ditch. A rock armored ditch over geotextile will have a long life but will have reduced capacity on lower slopes compared to concrete. Concrete ditches on steep slopes will need engineered energy dissipation. Grass lined ditches will be cheapest to construct, provide some water quality but need max velocities of about 3 ft/sec to avoid erosion. Grass ditches may require maintenance (mowing).
Plan on calculating the 100-year stormflow rate for the basin, proving flow capacity using the Manning's Equation for channels, and providing 1/2 to one foot of freeboard to satisfy any regulatory approval.