Actually Muggles, I think ya summed it up pretty well! At least IMO!
Drummed leather is more pebbly, whereas biker leather is more of a distressed leather. Very smooshy and it has the watersnake trim. I much prefer the biker leather over the drummed, but there are a lot of others who love the drummed. Just a matter of personal choice really.
The Navy drummed leather is really pretty, and the black is nice as well, it's the grey that I'm not fond of at all. It does look almost exactly like an elephant. But hey! Exotics are SO in these days!
Well put, Stinker! I don't own any drummed leather JC bags but have seen them IRL several times in dept. stores. To me drummed leather looks pebbly as Stinker pointed out and maybe "bubbly" is the best way I can describe it, whereas biker leather is raised but more "stripey" perhaps even more of an "antiqued" look, like the leather on a well-worn bomber jacket. And we can't rave enough about the shiny watersnake trim - found only on Biker bags, not on Drummed as far as I know - which is an utterly amazing and chic detail.