Regular First-Team Football will lure England International to Liverpool
There is no doubt that if Young joins Liverpool he will a regular start for the team when in form and fit as Liverpool need his pace and width in the side. Whereas at Manchester United, Young will be competing against Valencia, Nani and Park Ji-Sung (to a certain extent Giggs) for a starting place in the team.
On the playing front, Liverpool are a team on the up and it is clear for everyone to see. Who would not want to be playing for a team which has a great future as compared to Manchester United who I believe will be on a downward spiral the moment Sir Alex Ferguson leaves the club. I am sure Ashley Young has seen the resurgence in Liverpool coupled with the presence of Kenny Dalglish who seems to be able to raise the game of any player (well almost any player with the exception of Poulsen) he gets his hands on.
Yes, Manchester United are able to offer Ashley Young Champions League football which Liverpool can not next season but I am sure by next season, Liverpool will be back in the Champions League. For now Young would have to settle for Europa League football next season which I am pretty sure Liverpool will clinch.
In a nutshell, there are more positives for Ashley Young to join Liverpool than Manchester United and the main driving force for a player like him should be to play regular football as his talent would be wasted on the bench at Manchester United.