Say I am Cho Gath top and am laning against say Garen, should I grab Mercuries Treads for later on when they have 2 ap and 2 ad due to the fact that they have Tenacity or should I just grab Ninja Tabi to help in lane?

Mercury Treads aren't chosen for their MR (while it's nice), they're used for their Tenacity.

Ninja Tabi's Armor is likewise nice, but unless you're rushing it early game for the lane, you tend to get it if they have two or more heavy auto attack champions and limited magical threats (a single AP, tank, and support, the latter two of which are dealing minimal damage).

This is an extremely situational question. Is Garen owning you in lane? How fed are the other ADs on the opposing team? How fed are the APs? How much CC do they have on the opposing team? How much Armor / MR are you planning on building anyway? Can you get away with building a chain vest now and eventually upgrade it into a Frozen Heart later while grabbing Merc treads? Can you grab a Null magic mantle or Negatron Cloak and work that into an item down the line and grab the Tabi?

In your particualr example after judging how much CC the opponents had I would likely grab the Merc's if there was a decent amount of it and most of it could be reduced via tenacity while grabbing a chain vest early if I found myself needing armor. If the opponent's were weak on CC for some reason I'd go Tabi if I found myself in a tough lane against Garen or those ADs looked like they might be a real problem soon otherwise I might sneak a Sorc Shoes in to press an in lane advantage. I'm NOT a Cho player by any means but I believe he likes Frozen Heart and Abyssal Staff so those might be places to go if you took the Sorc shoes early and find yourself in need of either Armor or MR respectively.

A couple of other considerations (not entirely sure if these are good ideas, discuss):

- You could stick to level one boots until laning phase is over. Invest the gold you save in early g/10 items. You'll likely have to farm under tower to avoid ganks, but the gp10 will compensate for any farm you miss out on.

- If you decide to build ninja tabi, consider investing in a tenacity item as the game goes on. Moonflair spellblade provides tenactiy along with some AP, or if your support has shurelia's you can possibly upgrade your philo stone (if you have one) to an eleisa's miracle.

You'd probably either start with cloth armor or buy it your first trip back and get a doran's shield or ninja tabi if he's winning and/or the jungler is also entirely physical (Lee Sin, say), otherwise you could build the cloth armor later into aegis or frozen heart. You'd probably rush glacial shroud regardless, though.