Coco gives faster growth on autos because the roots get more oxygen and you control every feed, but it also punishes a missed watering harder than soil does. In soil the plant has a buffer, which suits people who cannot check the tent daily, at the cost of smaller plants in the same timeframe. A grower ran the same strain in both and posted the results on https://turboautoflowerseeds.com, with coco finishing about five days sooner and a bit heavier. For a first auto I would still say soil.