Lately I face teams with mostly AD champions (or at least AD vs AD lane )as a top laner. Thus, i'd normally go Ninja Tabi. But mid game shows up, and I get cc'd to the point that I can't do anything. Ninja tabi would reduce the damage from the autos but Mercury threads makes the cc last less. Which would you choose?
tenacity works only on stunns and slow duration if enemy team has knockups or simply loads of ad's take ninja tabi if you need to rush mr take tanacity i personaly take tabi more often than tenacity because 10% less dmg from adc and any autoatacker is more than being stunned for 0,5 second less
Treads.
Say it with me. "Treads". Mercury would make a very bad thread, cause it would just melt and your clothes would fall apart.
For your actual question, though, it depends on the situation (doesn't everything?). I would prioritize tenacity over the AA-damage reduction of ninja tabi in most occasions. Generally, you can just get more HP or armor somewhere else. Also, Ninja Tabi don't really work against AD Casters as well, since they only reduce damage from auto-attacks. I would only get Tabi if there's very little CC on the other team, and they've got two marksmen on that full-AD team.
If they have either: 2 or more champions with hard CC, or: 1 champ with extreme CC (
, 
, etc.) then get Mercs. If they have at least 3 damage dealers who deal AD damage and they don't have enough CC to warrant Mercs (always look too see if they have CC first before considering AD or AP damage threats) THEN you can get Tabis.
first, build boots that will help you win your lane. if you can get ahead then you can either zone out or kill their adcs mid or late game.
second, dont be scared to sell your boots mid/late game to switch them for a different set. look at the team, if they have a good amount of cc champions then switching to mercs will help you chase or get away. if they are really dependent on their adcs to carry them then sticking with tabis will be fine. dont feel stuck with one set of boots, adapt to how the game is going. they make that sell button so you can change your strategies :3
hope this helped
I should note that the gold efficiency of Ninja Tabi and Mercury Treads are actually relatively low, and that if you want to buy them, you should generally prioritize their passives above all else.
If you plainly want Armor or Magic Resist, other items would do you much better.
