Quoting Areetsa C
Eh, for all intents and purposes they're the same.
Not. In. The. Least.
Brickarms is a third party accessory brand who create products that Lego cannot. Because Lego has a strict "no war" rule, so Brickarms provides parts that Lego is "not allowed' to create. Mega Blox is a deliberate copy-cat brand who stole the entire idea of interlocking bricks from the Lego brand. The main difference is that Mega Blox copied Lego and tried to create their own brand of bricks while Brickarms was made to expand upon the never-ending canvas that is the Lego system.
Basically: Mega Blox copied Lego, Brickarms expanded it.