I've been trying to decide for many months now and it's about time I made the purchase. I'm looking for something beyond my nikon 18-70 lens and can't decide which is more important, a 2.8 aperture or optical stabilizer. I'm comparing the sigma 70-200mm F2.8 and 80-400mm with OS. I'm choosing sigma because I can't justify the near $2000 price tag on the nikon. Does the fast aperture allow fast enough shutter speeds that I wont need the optical stabilizer? So far they're just hobby pics. I do a lot of outdoor activities (camping, hiking, army, etc) and hopefully will find a way to make money off them soon.