Who exactly gets to determine if they will or will not testify

[kacekwl]

before Congress with or without subpoena ?

[SHRED]

1. We have an Imperial Executive Branch

Untouchable based on results so far.

[FBaggins]

2. It's a mix

In a more traditional scenario, you receive a grand jury subpoena. Does that mean that you MUST testify?

Nope. There are more than a dozen reasons why you might instead petition the court to quash the subpoena. Even if that is denied, there are cases where you would still decline to testify... get cited with contempt... and THEN appeal that decision.

The point is that the grand jury does not have an unfettered power to force you to testify "because we say so".

When we're talking about Congress vs. the White House, it is further complicated by the fact that the two are supposedly "co-equal". That's why such testimony is almost always the result of negotiations between the two sides.