iPad Pro
Pro = Very fast, fair price (so far)
Cons = No desktop apps
Surface Pro
Pros = Desktop Windows Apps (a big pro)
Cons = Massive price, even $1700 i7 slower than an iPad Pro
Google Slate
Pro = ?
Cons = Massive price, even $1700 i7 slower than an iPad Pro, No desktop apps
I'm honestly stuck between these three as a replacement for the Galaxy Note Pro 12.2 for my dad. He's used to the android and google environments so the slate sounds like it would do everything he wants. But for some reason he doesn't "trust" mobile devices like he does a real computer (I know, he's got it backwards, but that's my dad), so the Surface Pro 6 seems like a good fit. I think the iPad Pro is actually the best solution for him since he can still run google apps, and it is generally more secure and stable than the surface pros and W10. He's not a fan of apple though...
Right now I'd say the SP6 is the front runner, but I really want to see the new iPad Pros before we make that investment. Ugh. I really wish the larger format Android tablets would have stuck around.