iPad wins here because you can do so much more with it. Android tablets are oversized toys. With iPads you're nowhere near as limited. My neighbor uses an iPad in his studio, has an XLR studio mic dock hooked up and even runs a Bass guitar to it. He replaced a $1200 piece of dedicated studio equipment with it. And the iPad actually does more. Even though it's technically doing something it wasn't even designed or meant to do. Go to any B&M electronics store and see the shit you can buy for an iPad and look at the section for Android. I'm not talking cases and screen protectors, I'm talking shit like what my neighbor bought. It cost him about $350 iirc, which is more than a lot of Android tablets run. But it turns his 2 year old iPad 1 into something completely beyond what a tablet is.
Now him using an iPad to product music in a studio is obviously only going to account for a fraction of a percent of owners, but the fact you can do shit like that, and so much else makes it more appealing to the masses.