If the garden is small, a 150W lamp is perfectly sufficient, especially if it's just for security as a thief looking up at a 150W lamp shining down on him will not really know if they are lit up like a xmas tree or not. A bright light is a bright light sort of thing and if it activates a lot, a 150W lamp is obviously much better from an energy point of view, as well as being a couple of quid cheaper to buy. I think also that 150W filament tubes (I don't think they call them bulbs!) last longer than 500W ones.