If I had my choice I would go with 300mm because of the f/2.8 providing that you already have a wide angle 18-80mm zoom with macro or similar. You can add a 2x coverter lens and double the focal length to your existing lens (36-160mm) and make the 300mm to 600mm. Converters are about $50.00 I have noticed that you may need to use a tripod if you zoom in 300mm or more to get a steady picture( make sure you get auto focus if your camera is auto)
Hope this helps