Dell haven't been able to provide a U2515H review sample, so you can blame them for that. However; I have done some simple testing with one and I'd advise referring to TFT Central's
excellent review. It depends on your requirements. Gaming and general purpose? The AOC gets my vote simply because it's cheaper and I prefer the slightly richer look afforded by its gamma behaviour. The gamma tracking is more accurate on the Dell, so if you also edit photos etc. and don't own a colorimeter then the Dell is a better option.