Let's start with the green one.
First of all, it should be noted that this is the fourth strongest unit in Heroes II (black and red dragons, as well as titans, are stronger).
Attack and defense are 12. Quite decent. Health is 200 (only the aforementioned monsters have more). Damage is 25-50. Excellent. However, speed is average (4). Cost is 3000 gold and 1 sulfur.
My opinion: a very strong unit, especially in the hands of a Might hero.
Now let's look at red dragons.
Attack and defense are 13 (impressive :))). Health is 250 (excellent!). Damage is 25-50. Speed is above average (5). And the cost is 3500 gold and only 1 sulfur!
My opinion: in terms of power/cost ratio, it's one of the best units in the game. The key is that you only need 1 sulfur to hire them!
Therefore, I believe that red dragons are very interesting, but green ones can also make the game.