It's really going to depend a lot on what you're doing with the database, if there are any other 3rd Party Products you'll be needing on that server and what versions of Solaris they are certified on, etc.
If there are no special requirements, I would have to lean towards recommending Solaris 10. There's always a balance between the benefits of early adoption vs the occasional stability problems of new releases. But having said that, Solaris 10 is relatively mature at this point and being on the latest release will save you all the future headaches of being forced to upgrade when the older versions are supported.
That's my 2 cents,
Tony