The lighter fiberglass has a fine weave, it's easy to apply and wet out. The highs and lows seen later when set will be easy to fill, to a smooth slick surface. When more strength is needed in a stress area, extra layers of thin glass will be bette. You will have the best of both worlds...Lighter and Stronger in the end. Build light should be the heart of any aircraft design, with strength in areas of stress.