With excellent typing boasting a large number of resistances (including the infamous Bolt-Beam Combo), Magnezone certainly gets more usage than Manetric. Further, it has very good special attack, as well as a great movepool complementing it (Volt Switch, T-Bolt, Flash Cannon, Signal Beam). Finally, it has awesome ablities in Magnet Pull (making it a huge threat to the likes of Skarmory and Celesteela) as well as Analytic, making use of its lower speed. Even Sturdy is useful, but not as much as the other two. It also has well rounded stats allowing it to be used both offensively and to take a hit when required.
Unfortunately, it fears a 4x damage from Earthquake and 2x from Close Combat, arguably the most moves in the meta game. This is remedied by Air Balloon, or a Shuca berry or Chepa Berry, but switching out to another Pokemon (Landorus Therian particularly comes to mind with Flying type and Intimidate) is definitely the better play. It is also slow, so using it is harder, but that much more rewarding.
Manectric on the other hand is lack-luster, and while it does gain a Mega, that too is outclassed by the plethora of others, and is generally considered a waste of Mega slot. Still, it has some options in Over Heat + Volt Switch and also Switcheroo for that element of surprise. With 105 Speed and Special Attack but shoddy defenses, it still functions as a glass cannon sweeper (except not nearly as strong as some other glass cannons), and is worth a try once. However, in the long term, using Magnezone pays off better than using Manectric.