Personally, believe the Premiership is multi-tierd.
Manchester United and Chelsea, clearly the Big 2, with Liverpool isolated as the next challenger - they are steadily improving, and are starting to make significant investments in players.
I believe Arsenal are not in decline, but in transition - and perhaps will now be challenged by Spurs, who again, have huge investment potential.
There are then a number of clubs, such as Everton, Newcastle, Villa, West Ham, Blackburn who would have top six aspirations.