Afternic, because it gives you more exposure to end users by listing your domain on all the popular registrars. Most people have never heard of DAN or SEDO, so they will go to a popular registrar like GoDaddy (which non domainers have heard of) and search for a domain, where your Afternic listed domain will pop up.
I deliberately voted for Alter to appreciate the excellent progress he has made within a shortest time. Alter is about the newest, but certainly not the worst on the list. Again, how can you place a year old marketplace alongside with Sedo, Afternic and other "veterans"?
So my vote to Alter is vote of moral support and encouragement. We need more completion, please.