You are designing an A/B test on Bing. When would you run a user-tied flight versus an untied flight, and what are the benefits and drawbacks of each?

At Bing, a user tied flight is an A/B test which splits users into groups of treatment and control. An untied flight is an A/B test which splits each search into a group of treatment and control. As the PM designing the experiment, when would you choose to run a tied flight, and when would you choose to run an un-tied flight?