In my limited experience, I agree with what has been said.  Walnut for items requiring more abrasion. 
I've tried using Novus as an additive and also Brasso but the best results I have had has been with Flitz tumbler additive and untreated walnut shell (which I also got from Wellwood).  The stuff Wellwood sent me was supposed to be fine crushed but had a fair bit of larger shell in that needed sorting.  It also had quite a lot of soft inner husk which was a pain to pick out. 
The treated corn cob media that came with my tumbler was only really effective on items that were already in fairly good condition.  I think you can also get untreated corn cob but don't have any experience with it.