It depends on what you want really. I got a physical copy, but at times I wish I had gotten it digitally. I bought KI digitally, and playing KI is just more convenient some times as I don't have to fumble with a disk when I want to play.
Probably the most missed feature of getting a digital copy though, is the automatic updates that you gain access to. When an update is pushed out, you won't have to wait to play as it will happen in the background automatically. Add that to the fact that you won't have to get up to change disks to play, and that might add up to a better deal to you.