.mov & .m4v technical advantages & disadvantages?

Hello, I am new to iMovie and Mac in general (using iMovie '09) and starting to save some films I have been putting together. I had assumed that .m4v was the highest quality I could save as but recently checked 'export to Quicktime' and saw that the bit rate was higher and find the image quality a little better even though the frame size is 640 x 480 as opposed to .m4v's 720 x 580. Mind you these films are from a digital still camera and not the best quality to begin with but it is still important for me to save them the best I can. I am also hoping I have all of my settings the best that I can set them as. I am actually submitting these videos to festivals and was hoping for some advice on the best way to save them and which will be converted to DVDs. Even if someone isn't sure or knows of a site that has this sort of information, I would really appreciate it. I just don't understand the advantages and disadvantages of saving as either of these files and would hate for something to surprise me somewhere down the line. I am also a little worried that I am saving these in a way that is defeating my intention of trying to make them look their best. Thanks for any help you can provide.