Cleanse is much more powerful than Tenacity. Consider the damage creep in the game currently; you can non-exaggeratedly be killed in a second in some cases (team fight, assassin, etc). If you get hit by a stun for 1.5 - 2 seconds, the value of removing it completely and being able to flash is much higher than instead reducing the stun duration to 0.75-1 second because realistically in that scenario, you'll likely still die.
As for Mercs over Sorcs, that's mostly a meta adaptation. In competitive play, mid laners typically clear waves and roam, almost robotic like. The incentive to harass, fight, and contest your lane opponent isn't very high currently, so Merc's ends up being a safer buy (You don't get as much mileage out of sorc's in a passive lane).