He faces two basic problems. The first is that he can't testify under oath. The second is that he must.
He can't testify under oath because he has left a trail of patent lies and shifting accounts of central events, in particular Comey's firing, which Mueller's team will use to destroy his credibility. Moreover, there is strong evidence that Trump already knew Flynn had lied to the FBI when he told Comey, "He [Flynn] is a good guy. I hope you can let this go."
Most ominously for Trump, he can't testify because he is in the dark about all of the evidence that Mueller and the grand jury have already — in particular from Flynn, who is cooperating with the investigation. Not knowing what Flynn has testified to, Trump will be unable to maneuver around it.
All of which means that if he testifies, he is unlikely to stave off the obstruction charges and quite likely to expose himself to some counts of perjury. And while his defenders in Congress might try to pooh-pooh the obstruction as old news, or the product of Mueller's bias, they will be hard-pressed after the Clinton impeachment to defend Trump against perjury charges.
But Trump can't stay quiet, either, because he has to provide a good reason for Mueller to stay his hand. That means on-the-record testimony — offering in his own words some persuasive exculpatory account. Presumably that would entail copping to Comey's version of events, and admitting that he has been lying all along (something that's hard to deny when you tell multiple versions of the same story), but insisting that he lacked the required corrupt intent for obstruction.
For example, he might try to sell the account that he acted to shut down the investigation not to shield himself, his family and his associates but because he believed it was a meritless witch hunt and a waste of resources.
Besides, if Trump tries to avoid an interview, Mueller is likely to seek a subpoena to force his testimony. And the courts are very likely to uphold the subpoena under the authority of cases involving Presidents Nixon and Clinton. Trump will have sustained a crucial, deflating loss at the hands of the judiciary, and his legal choices will have narrowed to testifying or taking the 5th Amendment.
So Trump has to testify to have any chance of staving off charges. But Trump can't testify because he will only add additional counts to the obstruction charges. Zugzwang.