AI starts with mostly undiscovered and previously unrecorded talent. It was in fact a criteria before, not sure now.
The voice invites contestants to audition, many of whom are already professional singers or teachers, front a band, or back-up for well known artists etc.
The voice initially appears to have better talent because the public is not let in on many rounds of decisions. Not a bad idea, but even with all the Sanjaya's and Lazaro's, the Philip Philips and other unworthy winners, Idol still offers more excitement of new talent being discovered and blossoming.
Idol used to have a country week, Motown, Broadway, American songbook and even Disco week. They put the singers to a bit more of a test of versatility and nerves.
The Voice enhances the contestants vocals and gives them ridiculous production values as if they are established stars and picks only songs that support the "brand" the coach is placing on the singer. They refer to all as artists, which is not only pretentious but stupid. The voice is far more open to gay and alternative singers and looks and they cover much more current material. A lot of excitement is created through spinning chairs, boxing rings, celestial choirs and moving sets. The judges each have a distinct role and it is more a competition between the celebrities. The contestants are really secondary.
Idol was for many years directly connected to Clive Davis and they are a huge marketing machine. There is long term nurturing and promotion. The voice is just a ratings grab, temporary home, like the circus in town. Idol before Nicki fink and Mariah Carey was something more than a shiny carnival. They are a franchise of recording artists, and not only the winners. (The judges are the disposables.)
Kelly Clarkson and Carrie Underwood are the most famous of course, also among the largest selling artists of the last 10 years. Chris Daughtry was a legit find too, and many Idol singers are working in the business. Jennifer Hudson is always mentioned. She did not deserve to win over Fantasia and her voice though large in range and volume had the same problems then as it does now. She got lucky and got a bit better later.
Fantasia though disliked by many was a far more original find and a clear star from the word go. She kind of messed it up professionally and personally, but was on her way to being as successful as Clarkson or Underwood.
Clay Aiken, has sold millions with his mile wide vibrato and Divo voice. It is a good voice technically.
They have had some good singers on The Voice and certainly now have better judging and critiques but the contestants are too packaged, mostly too old to stir the necessary excitement, and Itunes is such a fleeting way of measuring success. Not quite legit.
Many famous singers and musicians follow and pay attention to American Idol. The Voice, not so much.
Idol should pack it in after Candice Glover wins this year and count their blessings to have a true singing star emerge again, with no help from the high paid help. They won't of course, but I will not watch again.
I don't watch The Voice because they dick around with and "assist" the contestants vocals too much. It is like Katy Perry, Rihanna week, every week.