That's a question that would require a very lenghty reply.
I would suggest that if you google "low votage versus line voltage landscape lighting" you will see a number of great sites that cover the topic.
In just about every aspect, the low voltage alternative is vastly superior.

Thanks. Your last sentence tells me what I needed to know.

120v must be installed by a licensed electrician, fixtures cost 2-4 times as much, less fixture selection, less lamp/wattage/beam spread selection, less decorative styles, wire must be buried 18' below grade and installed in conduit (lots of trenching and destroying yard), connections made in junction boxes. Project cost 5X more with few advantages other than you can use HID, mercury vapor and other super high intensity outputs (but this is only good for commercial properties) and that there is not voltage drop on long runs.

Low voltage is the opposite of everything above.

Here is a link that might be useful: outdoor lighting 12v vs 120v

We have the metal low voltage lights from Home Depot - love them. When a light gets broken (generally because the battery operated jeep driven by a 4 year old has run over it), I can replace it myself (this has happened several times) :). Probably saved several hundred dollars on electrician calls because of it.

The only benefit of the 120v lighting that I can think of is that you can put a plug on it, which might come in handy for using an electric blower, or plugging in christmas lights, etc. away from the house.

I put in an inexpensive set of low voltage lights along my sister's walk. The set came with little 10 watt bulbs in the fixtures which allowed the fixtures to be seen from the street and make a show, but I thought they did little to actually spread light on the walk. When I did my own yard I chose 120v fixtures because 1) I could use less fixtures to cover the same amount of area 2) I didn't have to worry about voltage drop on long wire runs 3) I can add fixtures without additional transformer requirments. 4) I didn't find the line voltage fixtures cost any more than the low voltage one's in the same quality range.

It was a DIY project with a lot more doing with the shovel. Conduit, concrete collars, 2' trench. FWIW I chose to put in 22 watt flourescent fixtures so I could dusk-to-dawn them without breaking the bank.

I used fixtures from this outfit and put the link here so you can compare low to line voltage fixtures.

Here is a link that might be useful: vista lighting