I can only judge on what has transpired and occurred. While one stands to be a greater asset long term, it's dependent on various factors and their harmony.
From what constitutes Pep's era so far, my pick is Dani Alves.
While pep's signings took some time to get going, Alves was the only one that never looked out of place from the offset. He immediately struck a chemistry, almost a telepathic understanding with Leo. That for me, was worth half his transfer fee. The Right flank Dani-Messi magic, in my opinion, was the herald of the Pep era. Since, he's been integral to our tika taka, and how we operate. The mastery and authority he offers week in week out on that right side, is indispensable. Barcelona, as an attacking unit is easier to contain without Alves, than with Alves in the side. We've coped without our other star absentees, (Abidal and Masch have filled in admirably well for both Puyol and Pique) but i very much doubt if anyone can replace Alves for a considerable period without a significant drop in quality.
Barca and D.Alves is the perfect match. And perfection doesn't 've a price.