Assuming you are talking about two products that serve a similar purpose:
Whiteboard paint is, well, paint that you can apply to a wall to turn it into a whiteboard. Whiteboard wallpaper is essentially the same thing, but in wallpaper form.
The main difference between the two is probably going to be application. Paint can be a bit tricky to get even if you're not experienced and one wrong move could mean starting over or having an imperfections. Wallpaper is generally easier to apply evenly, although it might take longer since you have to be careful not to rip or tear it.
Another consideration is price. Paint will generally be cheaper than wallpaper, but wallpaper might last longer since it's thicker/sturdier.