There was a thread not that long ago about this. Before seeing that conversation I had always assumed (like you) that the loss of his memory was the boon. Apparently most people seem to think it's the curse, from what I've read. But I looked over the text as best I could and didn't see anything conclusive either way. I expect it will be answered in Oathbringer.
My assumption is based on the fact that Dalinar never seems very upset about the loss of his memory. You might say, "How can he be upset if he doesn't remember her." Obviously if all memory of his wife has been stripped away then he can't fully know what he's missing. But that just doesn't fit for me. He doesn't remember her or have any memories of her, but he knows that he loved her a lot and he knows that he loved her well. If someone shows up and tells you that you have a child you have never met then you're probably going to find yourself in an emotional hurricane. Now imagine how it would feel if you know (objectively) that you spent a number of years raising the child before someone erased all of your memories. I, for one, would be pissed. Any time I was reminded of this child that I have no memories about, I would be oozing with bitterness towards whoever took my memory away.
The ONLY way it makes sense as a curse is if the boon is so very vital that Dalinar has come to accept the necessity of the curse. But even then... Just makes more sense to me that Dalinar loved his wife so much that, when she died, he went to the Nightwatcher to ease the pain. Probably didn't expect to loose all memory or the ability to hear her name... But I can see how he would walk away thinking, wow this sucks and I am such an idiot, but I guess I got what I asked for.
Jofwu's post in Dalinar and the Nightwatcher was marked as the answer