Well, the exp share and power brace operate differently, but they both concerns the EV's.
Since we're talking about BW2, then exp share is a held item. Pokemon holding an exp share not only splits exp between the party Pokemon and those holding the exp share, but all Pokemon receiving the exp also gains the same amount of EV's as the fainted opposing Pokemon yields.
For example if your going into a wild battle with a rattata, and you have a Pokemon in your party (not the leader) with an exp share, after defeating it, both the leader and the exp share holding will gain 1 Spd EV. If the pokeon holding the exp share is your leader, you wont gain double the amount of exp or EV.
But, the power bracer works a little differently. Since a Pokemon cannot hold an exp share and the power bracer in BW2, then the Pokemon with the power brace must be your leader for it to work. The power bracer yields 4 extra Attack EV's, regardless of the opposing Pokemon you faint. Back to the rattata example, the Pokemon holding the power bracer will get 4 Atk + 1 Spd for defeating the rattata. However, if the fainted opposing Pokemon yields Attack EV's such as Krabby, you will get 5 Atk EV's (4 from Power Bracer + 1 from Krabby).
So, to summarize and answer your question, Pokemon holding an exp share and a power bracer will significantly receive different amount of increased EV gains.
Hope I helped! :)
