The second formulation sounds easier to use to adapt to specific use cases too: just bump the priority of a message based on your business rules to make it more likely that interesting events get to your log database.
You could do (category, random priority) and then do lexicographic comparison. That way higher categories always outrank lower categories.
But depending on what you need, you might also just do (random priority + weight * category) or so. Or you just keep separate reservoirs for high importance items and for everything else.
But depending on what you need, you might also just do (random priority + weight * category) or so. Or you just keep separate reservoirs for high importance items and for everything else.