Souray is an expensive older defenseman on the downswing. Coburn is a young defenseman who has already shown he can be a competent number two even if he is having a down year. On top of that he's going to be cheaper.
O'Sullivan is $1.3m cheaper than Hartnell, but Souray's acquisition washes any cap relief as I demonstrated above. O'Sullivan is a little more talented than Hartnell, but he has his own issues as well.