The iPad has three advantages over a Mac: portability, the touch user interface (not to be underestimated), and price. Though there is some overlap on the last. In all other areas (computer power, available peripherals, and powerful apps) a Mac is the more capable device for just about everything, including music creation.
So the real question is if the iPad will be enough for what you want, or if you need/want and can afford a more powerful solution.
Both amateurs and professionals have created some amazing things using nothing but iPhones and/or iPads, but I don't think any of them would argue that a more powerful setup couldn't have done more and better.
Of course, if money is not a major obstacle, at least in the long term, you can start with an iPad and add a Mac and more gear as you grow into it. The iPad wont' stop being useful just because you get a Mac later. I've seen a few articles about musicians using then together to produce great music.
These are just my general thoughts on the subject. I'm not a musician myself, and have no significant experience with the recording or production of music.