hey guys in new to this forum and I have a delima I just sold my alpine 7863 and now I havea chioce between an alpine 7873 a 7875 a 7893 or a 7977 my question is whats the diff. between the 7873 and 7875 and 7879 and if everythinsgs the same should I get the 7977? its about a year old but it...