This post is a bit old but note that the iPad has a pretty short lifespan, I am a digital artist . I must tell you that you could probably get 2-3 years of smooth operation out of it before it starts to slow down, if you're getting the Pro models, that's around a grand every 2-3 years.
A XP-Pen Artist is peripheral - separate from your computer which means it'll operate as well 5 years later like it was the 1st day you got it. Not to mention, iPads probably won't go above 13 inches max. Most artists will tell you screen real estate is pretty valuable, to be able to draw without zooming in helps keep the colour, tone or detail work together in a composition. Though if you do buy a XP-Pen Artist , probably best to get something above 20 inches. such as my XP-Pen Artist 22E Pro Pen Display Monitor tablet .