I can think of one good way to find out!
You can think about the difference between
.qzvs as being the difference between data objects and visualizations of data. It's possible that a visualization has the data baked into it, but the data object itself should be a reliable source. Whether that data is formatted in the way you want it to be is another question. For example, the .qza might contain raw data, and the .qzv might contain summary statistics about it that you'd have to recalculate yourself if you started from the raw data.
Some visualizations have "click to download" links in them, to give you access to images or summary tables they contain, often in editable formats like
.tsv. Others have options for customization within the vizualization - for example, the ability to manually adjust a viz's Vega spec. It's always worth looking closely - you might get what you need for free.