Any opinion regardless of how much they contradict is still just as valid as any one else’s. The difference being is the source of information for said opinion.
You can say "I believe that the sky will turn purple next year." Of cores people will think your nuts, but that’s your opinion. The question of opinions validness is based on the information that brought you to this thought and opinion. Once you bring forth this information to others is when you bring its validness to question.
Now another person will bring forth their opinion as to why the sky will not turn purple next year. When comparing the evidence and information that brought about these opinions is where you test their validness.
Now if both sides do not bring forth any information as to why they believe the sky will or will not turn purple next year, then both opinions are valid as neither side brought it's validness into question by divulging the information that brought you to this opinion.
This comparison is where it turns into a debate. Debates are brought about by sharing each others opinion by giving information on said topic, and in the end each a conclusion as to which one has more validness then the other (who is right or wrong) where both parties eventually come to an agreement or comprimise on who has the most validness based on evidence and information.
The difference between an argument and a debate is rather clear. Both involve the expression of passionate feelings for ones own opinion or those who share the same opinion, how ever in an argument is where people let their feelings judge the validness of their opinion rather then fact and information. In which case, the person or both parties who judge their opinion based on their feelings rather then evidence and information are invalid.
Both argument and debates have strong expressions and feelings for their sides opinion, the difference is that in debates, the validness of said opinion is based on evidence and information, where as in arguments its validness are based on feelings.