It's clear that if the Thunderlords have a chain lightning attack, then the Sky Maidens won't. Some things are obvious.

The Thunderlords could be given a chain lightning attack, while the Titans retain only the single-target attack, but with increased damage. You could make the Thunderlords like "chaos units" in a figurative sense, giving them a relatively dangerous chain attack for their own units and a large damage spread, while the Titans would be ranged attackers, dealing stable damage to a single target.
Or, as a crazy idea, you could remove the ranged attack from one of the upgrades (while increasing the speed). That would be interesting...