The Thor is a monster. Hundreds of tons of high-density steel, two legs, four arm-mounted particle beams, a further four artillery cannons secreted away within its excessive shoulderpads and a really bad attitude make it the perfect vehicle for blowing through slow-moving enemies. But it's not invincible.
The Viking is a terror. It's a smaller mech unit armed with two giant gatling guns that can cream infantry in just a few moments. And it's oh-so-manoeuvrable. At any moment, it can fold its legs away and take to the sky. It makes the perfect base raider: fly it round forward defences and set it among the workers and drones for massive economic damage. But it, too, is not invincible.