Get what is going to be most useful to you. If you need a laptop for other purposes then a laptop can run DayZ just fine. I have a laptop with an i7 with 16GB RAM and a NVidia 555m with 2GB VRAM dedicated. It runs DayZ perfectly. But if you do not really need a laptop then a desktop PC is definitely the way to go. Cheaper and easier to upgrade, plus it is always nicer to game on a bigger screen.