Yes, of course fish is the actual difference. Although you can obviously just stock a pond with fish anyways.
>Also, your irrigation system must be fine-tuned to deliver just enough water to the plants you are growing, so it doesn't drain to other plants that you did not have any intention of watering.
No it doesn't. Even in drought areas the amount of water needed to water a garden is negligible as long as you don't make the mistake of leaving it bare earth.
>In any case, this is a moot point, because most gardeners do not have irrigation systems
Virtually all of them do, they are called sprinklers.
>And if you're just starting out, there is no guarantee you will have good soil
That's true, especially in modern subdivisions where they scrape off the top soil, sell it, and then back fill with clay excavated for the basement, leaving you with no soil and a thin layer of sod that needs constant watering to survive. But I think it is still cheaper to just do a normal garden. All you need is some organic waste to pile up in layers, no need to try to fix the clay below, just ignore it: http://organicgardening.about.com/od/startinganorganicgarden...