Depends on what you're after. If you want shiny bright cases, likely corn cob, if you just want quick and clean, likely crushed walnut.
If you want to extend the life of either of these media, try putting used bounce sheets (I cut mine into 4 strips) in with the media, it will collect a lot of the grit.
As for things stuck in the primer flash hole .... welcome to the world of re-loading!! lol
I use a dental tool, the scraper that is used for claening your teeth (ask your dental hygenist for used ones next time your in), they seem to work OK for geting the walnut out of the primer flash hole. As you guessed, I use crushed walnut, medium size. Got a 50lb bag at the parts store, they bring it in for sand blasting type applications. Dusty to start with but th ebounce sheets soon get that under control.