I thought the upgrade on dragons actually looked pretty favorable for god-killer decks before, assuming you had fully upgraded pillars; even at half upgraded (plus shields/lobotomizer upped) getting enough quanta is rarely an issue for me, and they offered more bang for your buck long-term (18% more cost, 25% more damage; against damage-reducing shields, which are uncommon in PvP but several gods use, their relative advantage is even larger). Now that the base dragons have been nerfed, it seems like a no-brainer.

Immortals probably won't work well against the gods--any damage reduction and you can kiss your chances of winning goodbye, and even without damage reduction you've still got a long, slow fight that will probably run you out of shields and possibly deck you. In PvP they're definitely the way to go now, but I think people tended to overestimate dragons compared to immortals in PvP to begin with.