prefered paper

By "learn" do you mean dots and dagger strokes or full pieces. You never really stop learning. Just for throwing strokes around I use plain copier paper. To do pieces on, even if they suck, I like Schoellershammer 4G. Canvas is nice because you can just cover your practice pieces with a coat of gesso and start over.
Well if your just practicing the basics I would use a sketch pad with the newspaper type paper. Cheap and works good to practice your brush strokes on. I use a poster paper I got at Michael's. But I usually try to paint on a metal surface. I found the metal 'For Sale' signs and 'No Trespassing' signs you get at Wal-Mart are cheap, like 90 cents, and good to prep like you would any automotive surface. I scotchbrite then primer, paint, airbrush,and clear. I know you asked about paper but thought I would throw it out there as another option. They have holes in the corner as well to hang your art up.
I never tried illustration paper and I know the 4g is specifically designed for airbrush work might have to give it a try soon the series I'm working on I will stick with claybord but after that I am gonna get a 25 pack of the 4g
When I use regular copy paper it causes a splatter pattern when I open up the needle a lot. I'm guess this is because I ussually use like 40 psi. I started using paper towels and they work a lot better at absorbing the paint.

Btw using iwata siphon feed and trying to learn to make shirts so that's why I use high psi
If ur just messing around get a cheap a3 sketchbook, and for more serious work i use canvas, the cheapest there is, they need a lot of prepping though.
I have been practicing like crazy since just starting out and have been using cheap copier paper. However since they are normal 8X10 sheets they fill up fast before having to grab another sheet. Larger sizes come in fewer sheets and slightly more money. Something I am considering doing is buying a roll. I have found some really cheap 100-200' rolls online, around $16. That way I can just tear off larger sheets and go to town. We'll see how that works out but I thought I would toss that idea out there.
Anything ya can get cheap is a good practice surface, even metal..Cut away a section out of an old washing m/c or another white good..Paint your practice strokes, keep some thinners beside you and when ya finished wipe it of with some thinners and start again..Like a whiteboard for paint practice, use it as many times as ya want.. :) paper wise i just use A4 reams of printer paper for my students, I use a large backing board and have them tape up/pin/bluetack 8 or 10 sheets at once so they don't have to continually stop practicising, butcher paper is also a good option as it comes in much larger sheets....GL