Short answer: they are both effective, but for two different reasons. So I wouldn’t think of them as either/or.
Long answer: as well as the thread @d_diston referenced, I would add this:
Your PD curve shifts positively in two ways: up or out. The first way is to progressively go for more power (curve goes up); the other way (less talked about) is to go at same power for progressively longer time/intervals (curve goes out). Even though you’re asking about SS, you can apply the more vs. long concept to Zone 3, Sweet Spot, Zone 4, and even lower Zone 5 workouts.
If you look around on the forum, many folks talk and ask about scenarios where they may progress from 2x20, to 3x15, to 4x15, to 3x20 on a workout by workout basis. They are looking at their total time in zone (40 -> 45 -> 60) and progressing it. Right after they decide to progress to a longer TiZ, they may chop the total workout up into shorter intervals in order to complete the workout. You gain similar fitness with the chopping up in terms of the targeted physiological systems (which is your question), but you really want to succeed at that workout and then progress it. <— “extensive”, curve goes out.
This is a different (and also effective) approach to the more common mindset: “ok I can hold this power for this long, let me see if I can hold 10 more watts for same amount of time”. <---- “intensive”, curve goes up.
If you really want to take a deep dive into the concept, look up the WKO4 videos on Fatigue Resistance and Time to Exhaustion (TTE). Tim Cusick calls what you’re talking about the “rule of 2x20”, and it’s insightful. What he’s getting at is most ppl fixate so much increasing power at FTP that they neglect how long they can hold power at FTP (TTE).
Both make you faster.