What is the difference between a 500watt & 250watt atx power supply?

What is the difference between a 500watt & 250watt (ebay) atx power supply, assuming both are new, and asside from wattage & price, both are the same?
In addition what is the difference between these two products I found? (ebay #1) & (ebay #2)…
and does anyone know of an ATX or other equivalent, or better power supply they can recomend for me that is $25USD or less including shipping to this location: 18929(ZipCode) PA(Pennsilvaina), U.S.A.

Thanks

ok thanks for the info 1 other thing what exactly does the wattage mean when talking about atx's?

What are you planning to do with it? I have LED based projects that I power with 5V and 12V wallwarts such as these http://www.mpja.com/prodinfo.asp?number=18520+PS http://www.mpja.com/prodinfo.asp?number=18469+PS http://www.mpja.com/prodinfo.asp?number=18027+PL

Check that you get 5.5/2.1mm center positive connectors. or an adapter http://www.mpja.com/prodinfo.asp?number=18238+PL