I like the Divine Guardian better now, but it still seems to overshadow the Dragon except in stall decks.
On average, Divine Guardian only deals 2/2.3 damage per turn, unless it ignites; it's better for healing and quanta denial. I suppose that Divine Guardian can be slightly faster than the Dragon if it ignites on its third turn (instant 20 damage), but if it doesn't then the Dragon would have already dealt 30 damage where the Guardian only did 26/27 (assuming it ignites on the sixth turn). In the long run, and it doesn't have to be stall, Divine Dragon is a far more powerful offensive creature than the Guardian.
Damage is easy, utility is not. Guardian gives any deck healing, quanta denial, and Unstable Gas. I'm not a fan of a divine being randomly exploding, but that aside, it heals an average of 2|2.3 per turn and denies 2|2.3 quanta per turn as well. Although, in terms of damage, +3|+3 a turn from the Spirit is as good as the Dragon but it also has CC. I'd rather have either of those much more often than I'd want a Dragon, I think.
What are some matchup examples other than stall versus stall that would make the Dragon more useful than the Guardian or Spirit? I guess I also don't know how long an average game lasts.
I would suggest that the Dragon start with 2 ATK and have 2 less HP.
I'd actually say the dragon is fine as it is. You change would cause it to do slightly less damage in the long run, which I don't think is necessary.
Matchups where I'd consider the Dragon better:
Counterstall - Some decks can heal a huge amount of HP each turn unless disrupted by a huge amount of damage, with Dragon being that source of damage. I'd consider a modified Dragon/Chimera OTK deck to be a good example of this sort of deck. Guardian merely hinders this deck in most cases due to the fact it has to heal you and deny the opponent for 2 turns before actually cuasing damage.
Rushed Acceleration - While it takes a bit longer to get out than an accel'd armagio, Divine Dragon has far more damage potential than an Armagio w/acceleration and there could be decks that attempt to get the Dragon out as fast as possible. Divine Guardian fails in this aspect of damage since it's damage pattern is unreliable and might explode on the third turn, thus defeating the point of sheer damage.
Any deck that uses Jade/Reflective Shield - Divine Guardian's damage is spell damage, and thus the only way for it to damage the opponent would be to explode. Divine Dragon on the other hand completely bypasses any shields.
Dragon is better at overall damage potential, while Guardian focuses much more on stalling and holding the opponent off. They both look pretty good to me.