Power Dragons are OP, Energy Cannons are really good as well, both better against SAV units than Nails
Energy Cannons are more or less upgraded Sentinels, however SAV units in general are harder to fight against so you do take some more losses than you did with Sentinels vs SAM units.
Power Dragons with their power shot ability (can hit flying units, ignore other defensive abilities like reactive armor) and their range are really powerful to fight with. 1-2 Power Dragons + Rogues can easily get through entire GE and get you to high attrition GBG, with minimal losses
In GvG they'll be able to easily handle all units if you have decent attack boosts, even Energy Cannons, and especially annoying units like Hover Tanks
Energy Cannons are much quicker to unlock so you'll probably fight most with those in the beginning (as well as Nails), but I'd try to unlock Power Dragons as quickly as possible, they're really good to fight with