Cyclopedia: What is a Derailleur?
Derailleurs are the “gear changers” that are used to shift the chain from one gear to another. By mechanical standards, they are pretty simple designs.
A bigger factor is the controls that are used to operate the derailleurs. Some controls work by pushing levers, others work by twisting a grip on the handlebars. Try to test ride each style, and see which works best for you.