Say I had a two hour workout planned today, but life gets in the way and I only have one or one and a half hour available, would I choose:
a) a workout that works the same zone(s) with the same intensity factor, but shorter in duration.
b) a workout that works the same zone(s), but with a higher intensity factor to compensate for the lack of duration.
What would you do/recommend?
Thanks
Use the “alternates” function and choose one suited to your available duration 
Depends on the type of workout, if it is high intensity / difficult - then try find a replacement within the same category which has similarly workout level (preferably using the alternates option).
If it is lower/medium intensity / easy or moderate, then increasing intensity to reach same TSS/kJ/WL for a shorter workout may result in transitioning into a training zone/stimulus different from the intention of having the workout in the plan to begin with and might also require more recovery than the original workout.
I think IF/duration works just fine for most workouts (especially at or below FTP). But as above, don’t make an endurance workout a threshold session! If you’re not comfortable to eyeball a workout profile and/or need an easy way to filter, then progression levels could be pretty handy.
In a silo, I’d go with option b, but it depends what the intention of the session is, how’s your fatigue, what’s coming up next etc etc. In other words, it depends…
This
c) Using “alternates” select your time and then choose a workout with a similar progression / difficulty. So if you are replacing a Sweet Spot 4.0, find something in the alternate list that roughly matches.
I think this approach is best as their logic (if I recall in my use of it) filters also for similarly structured workouts (micro bursts, short recoveries, etc…). Basically it pre-filters the workout list for you with the right options (if you try to do the similar task manually you’ll see all of the types and checkboxes to filter by).