the dragons will be more of a challenge on the ground if they front legs because they could run. also they would look bigger because they are standing on legs and not pushing on their wings. the wings would be on their back and they would have 2 pairs of legs

I don't know about being more of a challenging, but I most certainly would like 6 limb Dragons over 4, their wing slap is one of the most pathetic attacks I've ever seen, next to magikarps Splash.