Echoing what others have said, I prefer to use my Intuos at my desk, and I use Astropad when I'm working at a coffee shop or whatever. I like Astropad for when I have a constrained workspace and can benefit from drawing directly on the screen; if I have a lot of room, the Intuos works better for me, workflow-wise.
For me the biggest downside to Astropad is the lack of stylus hover support (which is a Pencil limitation); Studio sorta fakes it but not in a way that's actually useful to me.
Really it depends on what you're more comfortable with. If you prefer the 1:1 experience of drawing on a screen, Astropad is the better option. If you have a workflow that relies a lot on stylus hover or you really hate looking down at the tablet while you work or whatever, an Intuos is a better choice.
That said, "Why not both?" If you already have the iPad, then Astropad isn't that expensive, and you can try it out and then still have it as a tool for when you want 1:1 on-screen drawing. And you can use it as a stopgap while you wait for your new Intuos to arrive.
Also, in the iPad's favor, I actually find that its pen tracking is better than the Wacom, which still has a little bit of wobble. You can test this by putting a non-conductive (e.g. plastic or wood) straightedge on the screen at an angle and slowly draw a line against it. Wacom tablets - large ones especially - wobble significantly. Cheap Monoprice/UCLogic/etc. ones are even worse. The Pencil doesn't wobble at all, so far as I've seen. (That said this is more of an issue for someone doing precision inking than it would really matter for photo retouching.)