If so, which one do I rely on?

You might not have to choose, considering commit dd84bba64

When a job is complete, the controller will indefinitely update its conditions with a Complete condition.
This change makes the controller exit the reconcilation as soon as the job is already found to be marked as complete.

JobConditions is similar as PodConditions. You may read about PodConditions in official docs.

Anyway, To determine a successful pod, I follow another way. Let's look at it.

There are two fields in Job Spec.

One is spec.completion (default value 1), which says,

Specifies the desired number of successfully finished pods the job should be run with.

Another is spec.backoffLimit (default value 6), which says,

Specifies the number of retries before marking this job failed.

Now In JobStatus

There are two fields in JobStatus too. Succeeded and Failed. Succeeded means how many times the Pod completed successfully and Failed denotes, The number of pods which reached phase Failed.

• Once the Success is equal or bigger than the spec.completion, the job will become completed.
• Once the Failed is equal or bigger than the spec.backOffLimit, the job will become failed.

So, the logic will be here,

if job.Status.Succeeded >= *job.Spec.Completion {
    return "completed"
} else if job.Status.Failed >= *job.Spec.BackoffLimit {
    return "failed"
}