UA has very few planes with wifi -- currently only 3 747s and 30 A319/A320.
http://www.united.com/web/en-US/cont...i/default.aspx
The reason why the 747s are the first UA widebodies to get wifi is because they don't have AVOD in economy, just communal screens with a single movie playing. (Business and First have AVOD).
So I wouldn't say that UA is that much ahead of AC.