Which is preferable, subjective data or objective data?

In most circumstances, objective data is preferred. This is because objective data is more reliable due to its ability to be independently confirmed by a neutral third party. The recorded observations are also easier to recreate, as they're marked down by hard measurements.

That isn't to say that subjective data isn't useful, however. It's an important part of creating a treatment plan in medicine, and there are some observations that are inherently qualitative that can be a tremendous help in research.