I like Purcell to, but I don't value him above either one of Eberle or Moulson. Sucks to lose Niemi with your goaltending, but this is a deal too good to pass up. In most leagues a swap of Eberle for Niemi straight up would be a big win, especially in a keeper. There's no doubt in my mind who I would want in a one for one trade of Moulson for Purcell, and the answer would be Moulson. Value wise you won big, and in terms of it helping your team you won as well.

Eberle > Niemi
Moulson > Purcell