As for what changes from 42 to 50+... think of it as a continuous increase: Complexity of actions and tactics will go up. Hopefully you've already been leveling through the 4-man dungeons, so it'll be a smooth increase. Once you hit 50 it will keep going up as you go up the ilvl scale - largely on gear requirements though, complexity of tactics levels out at some point and just details change.

As for relic vs. myth armor... I'd recommend relic, though the myth gear is plenty important too. Please keep in mind that there is also i55, i70, and i80 gear in the higher 4-man dungeons that you can pick up as filler until you finish the myth gear. Also, coil turns 1, 2, and maybe 4 can be done easily enough and drop equivalent gear to myth.

Weapon is super important though and relic is easiest way to get a decent one. You'll invariably start at 50 grouping with people geared better than you. So weapon is the biggest thing that helps a tank gain agro -- you'll need it. In the relic fights you may feel like the worst geared and not contributing a lot. But there will be 2 tanks and the relic fights only really require 1 tank.