I recently acquired a 12.9" Apple iPad Pro + Pencil. With my recent cross-country move I haven't sketched a ton on it yet, but here are my experiences so far:
For reference, I should add that I've professionally used the Wacom Cintiq 12WX, 21UX, 22HD, and 24HD. My experience sketching on the iPad Pro has primarily been in the Procreate App.
As you might expect, the iPad pro has a beautiful display. When used with the Apple Pencil, the drawing experience is quite impressive off-the-bat. The calibration is pin-point accurate, and there is no noticeable lag. The friction of the pen on the screen is comparable to the friction of the basic black plastic nibs on the Wacom pen, which works great for me.
The iPad is light, has it's own OS, and is portable, which gives it a clear edge over any Cintiq that I've used - including the 12WX, which has a big power supply and a bunch of cables. That being said, I haven't used the Cintiq Companion which offers Wacom's technology in a self-contained, portable device.
Program-wise, the iPad Pro has the ability to use uMake, a 3D sketching program that I've written about here. uMake is only available on IOS, so that's one area that the iPad has a leg up on the Cintiq.
It is tough to get the hang of working without a visible cursor/brush tip on the screen, something that the functionality of the touch-capacitive screen of the iPad pro doesn't allow for. I'm typically used to working on a Cintiq which displays the pen's precise location on the screen, and the boundaries of the tool being used. That functionality is particularly helpful when using a large diameter brush or eraser, and I definitely miss it when working on the iPad.
The pressure sensitivity on any Cintiq is superior to that of the iPad Pro + Pencil experience. As a product designer who uses a more analytical drawing style (compared to someone who draws more for artistically), I can't say that it really bothers me. I tend to only really care that the line has some dynamism, and isn't just one thickness throughout. Procreate definitely has some pretty realistic pencils and brushes.
What I miss the most when sketching on the iPad is the lack of hotkeys. On the Cintiq, my left hand is in it's own world, traveling between the on-unit hotkeys and the keyboard. I couldn't even tell you half of the commands that I've committed to muscle memory, but they improve workflow and make sketching and rendering fast and fluid. I think that this is the most noticeable difference between sketching on a Cintiq, and sketching on the iPad Pro, and is the reason why I don't imagine ever using the iPad as my primary concept sketching device. That is, unless one of the iPad sketching apps includes an ability to set touchscreen hotkeys along the edge of the screen that are as easy to use as the physical buttons of the Cintiq and computer's keyboard.
At the end of the day, I think that my current setup is ideal. A 22HD on my desk as my primary workhorse, and an iPad in my bag for sketching ideas on the go (or streaming netflix). If you're a designer looking to get one digital sketching device, go with the Cintiq 22HD. You won't regret it. The 12/13" range is too small to be fully productive, and the 24HD is overkill.