maybe someone can give this poor, confused person a hint?

when I am adding a new feed to my bloglines account, it gives me two choices -.atom and .rss2 - which one am I supposed to choose? I've tried picking both at various times, so some blogs I get 'update' notices whenever someone posts a comment while others only when the blog owner actually adds a new entry. I can't, for the life of me, figure out which I've chosen for what blogs though.... anyone have any hints for me? I really don't need to know when 32 people have posted a comment to Gina Miller's blog :::roll eyes:::