I have recently got into wildlife photography, particularly small birds, currently use a Nikon 80-400 on a D300. I am looking to be able to improve but cannot decide if more pixels or a longer lens would be the best way to go. More pixels should allow tighter cropping, whereas the longer lens would fill more of the frame. Current thoughts are upgrade to D7200 (not considering D7500 because of lack of battery grip) or purchase Sigma 150-600. Would appreciate any thoughts.