If his lawyer has any ability to stop Trump, up to and including shooting him with a tranquilizer dart, Trump is not going to testify.
Once you choose to testify in your own defense in a criminal trial, you waive your 5A rights against self-incrimination. So once you say something untruthful in any regards, the prosecution can start calling rebuttal witnesses to demonstrate that you lied, which undercuts your entire case. Saying that you can’t remember when you get asked pointed questions makes you look really bad to the jury. Or, let’s say that he admits to having had an affair with Stormy Daniels (side note: is it really an “affair” when you’re explicitly paying for sexual services?, because I’m pretty sure the whole thing was strictly transactional); then the prosecutor can ask him why he lied in public, repeatedly, over the course of multiple years, about the sexual affair. Admitting he’s been intentionally and willfully lying in public for years is politically bad for Trump.
Testifying in your own defense is almost never a good choice.