Issa is doing a good job, but I'll let Congressman Frank Wolf explain why a House Select Committee is better:
However, the hearing also made clear that a thorough inquiry will require witnesses from across government – including the Defense Department, State Department, Intelligence Community, Justice Department and even the White House. Only a Select Committee would be able to bring the cross-jurisdictional expertise and subpoena authority to compel answers from these agencies.
His very informative letter can be read in its entirety here: Wolf Renews Call For Select Committee On Benghazi In Letter To Boehner
My thought is that Issa should continue for awhile as he has been effective in building momentum.
If the story continues and it looks obvious that requests for documents and witness are not going to be complied with...that would be the time to announce a select committee.
To do so prematurely will look like a witch-hunt to the public (from whom support will be needed for this whole thing to really go anywhere).