vs 
Discuss?
Edit: Cool, thanks for everyones input. What i got from it was "Unless it's Hard CC, just slows, Swiftness is fine. Merc Treads is great for additional MR for frontline non-chasing champions"
I typically buy the Swiftness boots on highly mobile assassins/bruisers or against a team with 4 or more slows.
Otherwise, Mercury Treads all the way.
I, personally - almost always pick swiftness. Treads has these two reasons to be bought:
So, as a top laner, the only time I see mercury treas as useful is if
A. If my Opponent is heavy AP (even then it is still debatable on how I'm doing in the Lane) B. If they have a lot of cc OTHER than a slow. C. If that cc is up very often.
If the enemy jungler happens to have these qualities, I may also consider Mercury Treads.
So again, as a Top laner, there is almost never a time where I personally would pick not swiftness (except maybe on Nasus, cause he tends to be the focus of the enemy TEAM cc, so its different, but still depends). Its just that most cc in that area are slows - Tenacity reduces the duration of the slow, swiftness reduces the size of the slow.
Now, in mid lane or bot lane, or even on a jungler, the check list is similar, but champions who meet the criteria appear more often.
Disclaimer - I'm Bronze, please don't judge me.
When you are Hecarim and have certain masteries in the defense tree
I actually seldom buy swiftness boots, if I need high mobility I go for mobi boots, if I need tenacity or MR, I'll go for merc treads. I can see swiftness boots being useful if you need mobility in the middle of a fight (mobi boots only help when you're not in combat) and if you already have tenacity (tenacity does help your mobility in a teamfight since you don't get cc'd for as long, and are free to move more often). Also if the enemy team doesn't have much hard cc to lock you down with, so you won't need merc treads.
Swiftness boots are great for chasing champions like Nasus and Hecarim. Merc Treds are better for tanks than need MR, and the tenacity is great as well. It's all personal preference tough, these are just my opinions.
Never.
Depends on the CC you are facing off against. If you are facing more slows then anything else swiftness.
If not merc.
I like boots of swiftness, becuase of the extra movespeed and resistance to slows. Usually if you were Udyr or something you would rather buy swiftness and Zephyr (because zephyr gives tenacity) and then you want AS or AD items depending on one's choice.
In addition to that, I consider Merc Trends if the enemy team has heavy CC or I am usually the tank who needs to be CC'd in a teamfight. For heavy AP champs I go for Spirit Visage/Banshee's Veil to mitigate the Magic they do, and for Damage comps I go Randuin's or Sunfire to ensure my strength is always up so I can beat them down.
In general I prefer Boots of Swiftness over Merc trends, but the problem with that is not always the heavy amount of slows the enemy gives for the return, but the amount of CC contributed that gives the slow more meaning than it should have.
I say, Merc Trends.
