Forums vs Yahoo answers clone

Which one is better for a professional network; Orca Forums or Answers Clone ???

You'll need to provide a little more detail about what you're trying to achieve as both are good in their own ways. It's impossible to know from your post which would be most suitable for your specific purpose.

I have a dental community site.... and i know that its wont be a good idea to use both forums and yahoo answers clone.

My thinking is that for discussions there are groups which have forums of their own... So is it better to use ANSWERS to answer doubts of users or forums will do?

We've been running the Answers Clone on our (big) site for around 10 months... with minimal activity. The forums are much more flexible in that open discussions and private discussions/threads can take place. The answers mod is a little limiting as you can only choose one answer... then that is that.