I'm going to be devil's advocate here because I'm not sure I agree with previous responses. As stated by Wanda in the comments, option 2 is clearly better for user conversion, but I don't think it's a better user experience, and I'll tell you why.
The user comes in, has a point to make, types it out, and often spends 10 minutes or more typing a response only to find out he/she can't post it unless you have an account.
At this point the user gets extremely frustrated with your site for wasting his/her time, and then leaves.
OR the user gets frustrated that you're now going to make them jump through a bunch of hoops, risk getting a bunch of spam mail, and having to remember another username/password combination which the user is likely, not interested in, but is being forced into, thereby wasting even more of the user's time in the registration process.
If you are asking strictly for giving users the best experience and providing the richest discussion forums, you should try to eliminate as many barriers as possible to allow them to post (within the technical constraints you're given obviously).