Strength means nothing. A weight lifter is "stronger" than an MMA fighter.

But as to who would win in a fight, read Brothers in Blood and Battle for the Cowl.

Nightwing should be because he's been in the game longer than Jason but if there is ever a battle to the death then only one of them is willing to go the distance.

I think nightwing because of him being an acrobat. It takes a lot of strength to do what he does.