If you're just doing regular touch-ups on edges that are already in pretty good shape, the stock ceramic SM rods (coarse/medium, fine, ef) should be fine for that. If you want to re-bevel any of your knives, that's where the diamond rods will be a lot more useful. Ceramic rods are great for finishing, but very, very slow at heavy metal removal jobs; they're just not aggressive enough.
If you just like very toothy 'bite' in your edges, the diamond rods can be great for that; especially with the high-wear steels (S30V/90V, etc.). Those steels respond beautifully with such an edge; a medium-grit diamond, like ~600 or so, is about my favorite for S30V.
David