I'm considering investing in either the Sigma 150-500mm f/5-6.3 DG OS HSM APO lens or the Sigma 50-500mm f/4.5-6.3 DG OS lens and would love to get feedback on either of these lens. I take a lot of sports pictures of my kids playing outdoor sports (field hockey and soccer). I already have the Nikon 70-300 lens, which is a very good lens but I'm finding that the reach is not good enough. The Nikon 500mm lens looks quite nice except that is over $8k and I thought that one of these Sigma would be a good compromise. I currently shoot a D90 and plan on staying DX format as I don't see the need to upgrade to FX (especially since I would lose 1/3 of my reach). Hoping to hear from others who have used either Sigma lens. Thank you!