DEF/ST ATL,SD,SEA,or OAK who to keep ?

[shadowfire36]

im having trouble picking which defense's to use. currently i have San Diego and Seattle. i picked up seattle as a flex play so i dont wanna drop them since they play detroit in week 5.
But im trying to figure out is i should drop either of my defenses and pickup oakland or atlanta for week 2 . i was considering to drop my san diego def and use oakland for week two against KC ,but at the same time i keep reading such high praise for Atlanta as long term defense .

so who should i keep as my two defense's and which to start for week ?