Lets not forget Joel also spent stints in the midfield and also forward over this career. I remember him winning a game or two of his boot. In terms of playing multiple positions he trumps Nick.
This is a key point for me.
Are we comparing Joel and Nick as defensive players, or for all round ability?
If it is all round, then Bowden is way in front. Nick was expected to develop in to a midfielder when he was drafted and didn't, but has become a superb backman. Joel could play forward, back and through the middle.
If we are talking as a pure backman, then yep, I'd go with Nick, although Joel's footy brain, calmness and leadership are very hard to overlook.