When would you use a "loop" in a list workflow versus "wait for item update"?
Advantages/disadvantages of each.
It's like everything in computer science: polling versus events.
When you poll (querying the status of an object in a loop), there's 2 drawbacks:
While with an event ("wait for item update"), you have no worries like that: your workflow really sleeps till the event occurs, and you're notified immediately (note: in the SharePoint workflow world, "immediately" means "with a 5-minute max delay").