I recommend an app called Idraw for the iPad (might be available on other platforms, not sure). It's a fantastic app, has some great features on it and most importantly you can save your files as .svg files which can then be bought into the professional vector drawing software Adobe Illustrator. In actual fact it has a lot of the basic stuff that Illustrator has. I've also seen some great animation based apps for the iPad, not bought any yet, but I'm sure there are reviews for these things dotted around on the Internet.
Although pricey, I would recommend going with the iPad, simply because, those designing apps for creativity would naturally think of the iPad as first port of call, and therefore you will have a greater choice of apps for animation, illustration etc.
To help answer some of the debate about why design is usually done on a Mac....
I've been a graphic designer for nearly twenty years and have used Macs since the beginning, this is because Macs were at the forefront of technology for the creative industry.
Up until recently the operating systems on Macs and PC's were very different. The Mac has always been intuitive, with the operating system set out in a very logical way. It is also easy on the eye and brain. Windows however, was an ugly, cumbersome and non-intuitive operating system, which for designers, was simply too frustrating to use.
Nowadays, Windows has emulated the Original Mac OS so much that they share a very similar look and feel. But the creative industry as a whole have stayed with Macs, because we know the OS so well, and it suits our way of thinking. Creative people generally, get urked by ugly things, and Windows has traditionally been an ugly and cumbersome system. Mac OS just worked very intuitively because it was created with an understanding of how their core users used their brain. We wanted sheer simplicity to allow all our time to do the creative stuff, not the functional stuff!
I know PCs have come a long way to become more user friendly and have many strengths. Most people in their day to day work just prefer what they are used to using. They already know their way around the system. Designers, generally speaking, just have an affinity with the Mac OS and feel very uncomfortable using Windows.
So there you go, my two penneth worth
