This is a tough decision which is why I haven't ordered the iDSD even though I almost pulled the trigger last night.
I do prefer top sound over anything else so the iDAC is my new leaning. However, if one can wait, there is a case to be made for the portable Nano iDSD and the eventual Mini iDSD coming out.
What a hobby this is planning out future purchases
I was looking at the same choice. I simply wanted the best sounding playback into a standalone headphone amp from the line outs in a desktop setup. Vince from iFi said the iDAC was the slightly better sounding unit in that scenario because the output stage is more sophisticated. I use it with the ifi iUSB power supply which is a notable improvement over being driven directly by the computer USB power. I difference especially when driving headphones directly (which takes more juice than just running into a line in stage.) The iDAC has a decent headphone amp.
All that said, I think the main selling point of the iDSD (other than it's greater format flexibility) is its internal battery for portable use. It was put to me this way:
best sound in a computer setup --> iDAC
almost as good sound from a portable (iphone/android) setup --> iDSD