After looking at both the websites, I will have to agree with what Siram said in his post published below. If these websites are intended to change the opinion of people in the opposite group, I don't think they have succeeded in doing so. It is so interesting that such vastly difference exist and both consider themselves to be truly scientific websites, created with no intention of profit. As Siram mentioned, the fact that there is still so much ambiguity around this subject as to whether we can concretely say that global warming is a problem or not, continues to impede our progress on solving it, if it indeed does exist.
I found that both the sites seemed to attack the other side rather aggressively, in their attempts to prove the other side wrong, and I am not quite sure whether that is the best approach. When you are trying to convince someone of an opposing viewpoint your own beliefs, it might almost be more effective to at least acknowledge the other side before you go into your rant of why your ideology is better. However, I will admit that they do a good job of going through all the so called "myths" of each side and disprove them one by one quite effectively.
If you are an ordinary person, who had no opinion on the topic, was to stumble on both of these websites right after the other, I'm not really sure whether they would walk away having formed an opinion. Both sites claim to be credible, scientific websites, not designed for profit, but rather to educate, which makes it even harder to judge how reliable they actually are.
Content wise, I think that WWF was more convincing, but it also might be because I tend to agree with what they say more than the other website.