Am thinking about making a purchase (sssssh don't tell HWMBO) and looking at these three options. I am going to be doing a lot of photo shoots and reportage work towards my level 3 city and guilds and thought this lens choice might be a good purchase. Obviously there is a significant difference in price between the Nikon and the others but wondering whether it is indeed worth the extra money. Any thoughts about the lens and their strengths and weaknesses would be greatly appreciated. Many thanks, Sarah