I had both Mac and Pc and I prefer desktop Pcs. Costs less, so you can chose a more powerful configuration.
Anyways, I think both Mac and Pcs are good for Creative Cloud, so choose what you like more.
I prefer big screens of course, but I think you can work fine on a 21". A 17" may be a bit less confortable, but it also can work (I known a great art director who works on a 15" laptop, and did awensome works)
I think a touschscreen would be a waste of money if you choose a big screen (I think they are expensive and probably won't help you much if you need to design). I suggest you a bigger non-touch screen and maybe a wacom pen tablet.
Hope it helps!
Cheers
Giovanni