Leo says it's depends. Cable is technically faster, but he would share bandwidth with others which can slow down if the office doesn't have enough bandwidth or if all the neighbors are streaming Netflix. DSL provides a dedicated line, but if he's too far away from the hub it's either slower or not available at all.
What about AT&T UVerse? Leo says that they send the same video information over the same pipe they send data. So he can kill his access if he overwhelms it.
It also depends on what he's using it for. Skype, for instance, works best with DSL. Netflix is better on cable though, generally. He really should find out from other people in the area on what's best there, because it really can depend on where he is.