Beams and rafters are suitable for small barns, give a more open ceiling, but are labor intensive and aren't practical for wide barns with long roof spans. Trusses are engineered to withstand the load for the specific building and sit right on the posts of the pole building or the top of the framed walls. They require less material and go up fast but due to their gridwork-like appearance, make for a less open ceiling. Metal roofing might work for you. Metal is much less expensive than shakes and safer in the event of fire in the area.
Shakes are the most labor intensive type of roofing to apply.
Metal lasts longer than shingles and rolled roofing. However, metal roofs can be noisy unless they are insulated. The panels will let in a lot of light and cost less than windows or an equivalent amount of barn wall siding. Fiberglas panels do not allow for air flow like windows. Fiberglas should not be used where a horse can reach it as it would easily break.