It depends. Are you getting harassed a lot and require that security you get from the health, lifesteal & movespeed from those items? or are you destroying them and want to destroy them faster?
If I have, say, soraka or taric healing me a bunch, I don't need to worry too much about the extra health. Also if they aren't harassing me a lot and I don't really need the lifesteal, then that works well too. The main thing is that with dorans, you have to sell them, so they're meant more for security in lane rather than just a core item as they don't build into anything.
You could, in essence, reach your safest point faster if you go bf sword first (you could get a really quick bloodthirster which is more damage and lifesteal than 2 dorans blades will ever hope to achieve.) The way I view it is this.
BF-Sword first: Weak -> Weak -> Very Strong
Dorans: Weak -> Not so weak -> Somewhat strong -> Very Strong
So, it's your pick. If you're doing well and can afford to go without, then do it immediately, otherwise, just go for the dorans. If you're questioning whether or not you should go for either dorans or bf sword, go dorans, because dorans are never a bad idea.