Look at the resources Redknapp has had compared to O'Neill in the transfer market.
When O'Neil took over at Villa, they didn't have the best team going and he had to find bargains like Laursen etc and kept building slowly, and now they are a quality side.
Whereas Redknapp took over a side, who yes were bottom of the league, but had players like Modric, Pavlyuchenko, Bentley, Lennon, King, Woodgate, Bale etc, it makes it a lot easier to turn around a situation like that with those sort of players, and also had money to spend in January bringing in players like Keane, Defoe and Palacios who are all quality players, but not to take any credit away from Redknapp, he's always been a brilliant man management expert, and it showed when he went to Spurs with players like Lennon and Modric turning into 2 of the best players in the league.
O'Neil deserves it more in my opinion due to the fact he's slowly built over the seasons, and although he's won no trophies yet they get closer each year, a prime example being this year that they lost in the Carling cup final, and they are in the semi-final's of the FA Cup. He's done a fantastic job and deserves a lot more credit than he gets.