SST is not a power training concept, though it's derived from lessons learned from training with power.
"Sweet Spot Training" is best defined here: http://www.fascatcoaching.com/sweetspot.html
Without a power meter, it's training at the point at which it gets uncomfortable to grab the water bottle for more than a gulp and pushing. Long SLOW miles doesn't exist. It's long steady miles, if you have time. I have 18 hours per week training time, so I can afford to do a couple days of long STEADY miles, L2/L3 (Coggan) in power, but what would end up being mid L1/L2 heart rate training zones (Friel defined).
If I was time crunched, those days would disappear in preference to solid steady threshold improvement. That means training for improvement of 1 hour power. 2x20 minute intervals are the top end of SST. The bottom end of SST as shown in the referenced article is shown as upper L2. But the reality is that for the time you'd need to spend in that range, you'd might as well move into the upper L3. It's not that much more fatigue for quite a bit less time.
In power training, we quantify the stress on the body based on "TSS" - or Training Stress Score. As listed elsewhere, 100 TSS in 1 hour would be a max effort 40km TT. It's would be an effort that would be felt the next day for most folks, but you should still be able to go pretty hard following a 100 TSS day. Run a couple of them together, and most folks will need a bit of a break.
2, 20 minute intervals @ threshold power ( your average heart rate would probably start below LTHR, but would end above LTHR by the end of the interval ) is "worth" 66.7 TSS. Mind you, 20 minute intervals @ threshold power suck big time.
In keeping with "sweet spot" mentality, 3 x 20 minute intervals @ 91% of threshold are "worth" 82.8 TSS. Most of the folks here would tell you that 91% of threshold is a world of difference in PE from 100%. 91% threshold power for me, with a LTHR of 177, puts me at an average HR of 165-168.
Based on TSS, one could argue that 3 x 20 minute intervals will benefit you more, with a lower PE penalty while doing them. Toss in 5 minutes recovery between intervals, and 10 minutes warm up/cool down and you have a great 1.5 hour workout.
One of my favorite workouts is to just pop it at 91% threshold and hammer for up to 2 hours. I'll toss in 3-4 VO2max "pushes" on climbs nearby to change up the pace with 30-45 seconds recovery on the other end and get both threshold improvement and VO2max improvement all in one workout. I love this workout.
My average heart rate has typically been 165-168 for these workouts, but I don't always wear the HR strap.
On the side notes, the last time I did this workout with VO2max stuff, my average power was 89.5% threshold for 1 hr 45 minutes, 3 VO2max intervals mixed throughout based on the route I was doing, and it was worth 141 TSS before pedaling on into the mountains for a couple hours. Avg heart rate was down to 162 for the workout, which indicates to me it might be time to bump the workouts up a bit and consider my threshold power to be higher than what I'm considering it to be now.
So the next time I did the workout I only did 1 hour, but I held it at 94% threshold power. Avg heart rate for the second half was 168, which is closer to what it should be on a regular basis. This is my indication that what I'm calling 94% might actually be 91% and I've just been sandbagging for a month.
End of the day, your best bang for your buck is being shown over and over by sweet spot training.