Meltdown is a medical sort of 'bot, not dissimilar from Ratchet, sporting heavy weaponry along with a specialized repair beam for team support. He can repel enemies, fortify himself while attacking, and boost allies after a quick shot of repairs.
By contrast, Duststorm is something of a vampiric sniper. She sports long-range weapons that can leave bombs, stun targets, or tear through shields, absorbing the energy of enemies that she demolishes. In desperate situations, she can also accelerate projectiles in close-range engagements to take out approaching foes. Check out the videos for both robots just past the break, showing off their skills in action as well as a glimpse of the game's battlefields.
[Source: Jagex press release]