Short answer. Neither automatically. If the petition is not resolved at the initial conference, it is decided in a summary hearing on affidavits, documentary evidence or oral testimony, at the court's sound discretion. Where testimony is taken, the judge specifies the witnesses and what they may testify about.
What the law says
If the petition is not resolved at the initial conference, said petition shall be decided in a summary hearing on the basis of affidavits, documentary evidence or oral testimonies at the sound discretion of the court. If testimony is needed, the court shall specify the witnesses to be heard and the subject-matter of their testimonies, directing the parties to present said witnesses.
Family Code, Article 246 — Summary Hearing on Affidavits and Evidence. Read the full provision →
The case may end before any hearing
The article's opening words assume a possibility worth noticing: the petition may be resolved at the initial conference. Many are. These proceedings exist to answer a single, concrete question, and once the judge has both spouses in front of him and the documents on the table, there is often nothing left to hear. Everything the article goes on to say about affidavits and witnesses applies only if that does not happen, which is a reason to arrive at the conference properly prepared rather than treating it as a preliminary.
The judge chooses the form of proof
Where a hearing is needed, it is a summary hearing, and the three modes — affidavits, documentary evidence, oral testimony — are available at the sound discretion of the court. That discretion belongs to the judge, not to the parties. You do not have a right to insist that a matter be proved by live witnesses because you would rather have them, nor can you insist on paper alone. Most such hearings lean heavily on documents, because the questions these petitions raise are usually documentary in nature.
Testimony is fenced in
The second sentence is what keeps a summary hearing from becoming a trial. If testimony is needed, the court shall specify the witnesses to be heard and the subject-matter of their testimonies, then direct the parties to produce them. So the witness list and the scope of each witness's evidence are set by the judge rather than assembled by counsel. A party planning to call a series of relatives to describe the history of the marriage should expect that ambition to be pruned to the narrow issue the petition raises.
Prepare documents first, witnesses second
Given how the article allocates control, the sensible preparation is documentary. Have the title or property records, the proposed deed, the marriage certificate, the proof of notice and any valuation ready and complete, since these are what the court can act on without a hearing at all. Prepare affidavits that are specific rather than general. And identify, in case the judge asks, the one or two people who can speak to the facts that actually matter — not everyone with an opinion about your marriage.