Top box - pretty secure, very waterproof, limited choice in terms of looks and boy can they look ugly on the back of a bike, the box is likely to come off easy but the rack will be pretty permanent and you need space to store the box when not in use. Tail bag - not so secure but pacsafe make metal mesh nets you can put the bag in, not always waterproof but it can be done with bags such as from Lomo, big choice of prices, looks and sizes, far more flexible in terms of storing when not in use and putting on and off the bike. If you leave your bike parked up in public a lot I would go with a top box, otherwise I would go with a tail bag.