I use this:
http://www.midwayusa.com/product/541...ProductFinding
It's Lyman treated corn cob (the green stuff). I tumble with it before size/decap - just like you plan on doing. I have no other media to compare it against, but it seems to work perfectly for me.
It's my understanding that walnut shell is more abrasive - harsh, if you will. I don't know if it's better or worse for the application.
I couple years ago, I set out to do what you're trying to do now, and this where I landed. I'm satisfied.