Which of these would be the bigger achievement?

Good question, its a well balanced consideration.

I'd go for Haye. It's far more historic to become cruiser and heavyweight champ. In those terms Haye would be remembered long into the future.

And okay, beating Valuev is not an impossibility by any means but beating not only one Klitschko but two is a large ask to say the least. There really are three giants in the way.

And look at it this way, Froch would be even money at worst against anybody in the tournament, christ he's favourite or second favourite to win it anywhere you look. Klitschko was a 1/2 favourite over Haye, and thats not even getting to Vitali.