It actually doesn't matter. For my cases, I found Mathpix to be much more reliable than Nougat, for example. So, when you have hundreds of documents to convert a year and little time for manual labor on the results, paying a yearly "pro" subscription fee is worth it. However, it will really hit your pocket when you need to prepare datasets from thousands of PDFs... That's what you can't afford without a budget allocation from your project.