I'm thinking of buying a longer lens so i can do sports photography and have pretty much narrowed it down to these two lenses.What im wondering is if it would be smarter to buy the 180mm lens because its faster and just add a 2x teleconverter to it whenever i need a bit more power or should i get the 300mm lens because it will be faster than the 180mm lens with a teleconverter.I'm interested in bird photography and sports photography like baseball and football so if anyone can help it would be much appreciated. FYI i use a nikon n70