I was thinking about this, and I think this question is one that plagues many cyclists. Most riders try to choose SMART. But what is smart? I would argue that it is the combination of efficient and enjoyable (or as enjoyable as it can be :)) I would love add healthy to that definition but refrain.
If you agree with my definition, then that means smart is very individual. What is smart for one person is not smart for another. Indoor smart and outdoor smart are different as well. A big part of smart depends on your goals. In common, current, popular recommendations - healthy and fit are not the same as maximum performance. My beliefs are similar with Dr Stephen Seiler’s (google him or polarized training) which brings these two much closer together.
If you believe in the TrainingPeaks Coggan math regarding fitness, stating the higher total training stress over a longer period of time (usually stated as 42-days), the more fit you are.
This Coggan TSS math is not perfect because this math says FITNESS = duration * intensity * days (not exactly, but that is the idea). In the most basic sense that would indicate if you have a fitness level (CTL or chronic training load) of 100 and are over-trained, you are more fit than if you are under-trained at 90. I realize the Coggan/TP math goes farther by brining in the idea of Form (restedness) but most people following this math try to maximize the CTL during the training phases, which is not necessarily SMART. Dr Seiler calls this chasing numbers. Chasing the highest CTL, in many cases, can produce reasonable results but is frequently not smart 
As studies have show polarized training at a lower overall TSS/CTL produced better performance results or measurable improvements than higher intensity sweet-spot training. Disclaimer: This is a generic statement and talking about a specific study that is not universally true for all riders.
Back to smart, if you enjoy sweet-spot workouts they are potentally SMARTER than doing lower intensity work but if you prefer lower intensity endurance rides that that is likely the better choice. You also should not do too much sweet-spot in a week and should have more variety to be smart.
To be smart you can pick what is better for you, here is a couple example choices
Here is a moderate intensity sweet-spot workout, which is 61TSS for 60 minutes.
https://app.trainerday.com/workouts/4x8min-sweet-spot-60-5
I would say below is an easier 72-minute (for me) endurance ride that is 59TSS
So now in the above you have made a choice based on Coggan’s math and picked a smarter workout for you. I am sorry if this is over-simplified as there is more complexity to high performance than this, but this gets you headed in the right direction to understanding what works for you.
Ultimately you need to find your own version of smart and if your smart has a strong focus on health first performance than I, Seiler and many others, recommend you have a larger distribution of lower intensity work 90% easy /10% hard based on time-in-zone or 80/20 or 1-in-5 of your workouts can be hard.
As you can see I am passionate about this subject… Love to hear others thoughts.