What Dhoni is fantastic at is playing a large number of roles. Can come in at 3 and basically kill the opposition (remember his first 2 sensational centuries). Can come in slog overs to set a big target or chase down 8-9 rpo. And can also build an innings from tatters. In terms of big hitting, he definitely scores over Bevan (that exhibition match aside)
The reason I've voted for Bevan is that he did it for little longer than Dhoni has done so far, and Bevan had hardly any holes. Dhoni did well in last tours of England and Australia, but hasn't done as much in SA. It's pretty close though.