I've seen phrases translated using both forms and apparently meaning the exact same thing.
For example, I have seen "I am busy" translated as both mi estas okupita and mi estas okupata.
How do I know when to use -ita instead of -ata?
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-ita means that something has had something done to it, while -ata means something is having something done to it.
For example, if I call someone mia amata filo, that means that he is my son whom I love, I love him now. If I call him mia amita filo, that means he is my son whom I loved, but I don't necessarily still love him now - significantly different meaning. And mia amota filo is my son whom I don't necessarily love now, but will in the future - a rather odd and seldom used meaning.
In the case of okupita vs. okupata, the difference is mostly one of nuance, and the specific sense of okupi. If you say mi estas okupita, that emphasizes that something has, as it were, taken over you, taken over your attention, your time, your energy, etc. If you say mi estas okupata, that emphasizes that something is currently occupying your attention, time, etc. So in this case the meaning is very similar, and the difference is a matter of nuance and emphasis, but in most cases, -ita and -ata are significantly different.
If the main verb or the context is in the past tense, -ita indicates something that was completed at that time, while -ata indicates something that was ongoing at the time.