Depends on the adc. If it is someone like Ashe who does tons of crits you could do either. It depends on if you want to do a ton of damage or if you want to take next to no damage from your fights. Crits are usually quicker fights and are good if you are taking out a group of people. And Lifesteal is better for one or two people and sometimes the fights take longer for you to kill them. I hope I answered your questions. If you have more questions PM me or reply. - The Cloaked
Depends. If you are stomping out a lane get attack speed and some crit, but if its even or you are losing, get the bloodthirster or blade of the ruined king. Other then that there are a few champions who should usually get a certain item (such as Vayne and Kog'Maw almost always getting blade of the ruined king Other then that you should try and expirement with each character and occasionally shift your build around. Never stick with one full build for more then a few months. Theres something called a META that shifts and you need to occasionally shift with it for the most benefit to you.
Right now I think the most common build paths for Quinn are either IE into SS or BotRK into Youmuu's. That is what I got by searching through high elo guidex at least.
The rest rush