I'm leaning towards atom but I'm not sure.
Why not offer both? It's not hard to do and you give users a choice between:
Then someone can choose which best suits their needs. If they're not sure, then it won't matter to them anyway.
But if you had to choose, go with RSS. It's by far the most widely used and the odds are your users won't know the difference between RSS and Atom and thus won't be using Atom's extra features anyway.