I would be inclined to say it depends on how your goal event is measured. For example, if your goal event is a specific race or route with x distance and vert, I would measure the progression by those parameters. If the event is more time based (like max distance/vert in a 24hr event) I would lean towards only progressing by hours of training.
In practice I do a bit of both… when I am in maintenance mode or coming back from a break, without a specific goal in mind yet I tend to just try to get a certain number of hours per week via several different modalities. As I define my goal better I start focusing on getting a certain amount of vertical + specific work in per session and per week.