It mainly depends on your overall fitness, ability to recover and experience with training. A more experienced cyclist will opt for the 3:1 program while the 2:1 program is more suitable to less experienced athletes.
There isn’t a clear tangible criteria to define which is best for you.
Basically it is related to physical ability.
In an ideal training plan we must push 100% and beyond when required, in order to improve, but if we are tired we are not able to do it.
For some 3:1 is too hard and 2:1 is better.
For some others, the opposite.
Just to add to this, there is also an age component - the older one gets the more recovery one needs so 2:1 may in fact land up giving a better outcome than a 3:1 where an athlete is getting insufficient recovery time. I’v seen somewhere the age of 55 as being one to consider 2:1 plans but I would imagine that’s highly variable.
If you’re like me, you sign up for a 2:1 plan, then on the third week you find all these cool new workouts (like the new On Location and Pro Rides) that you want to do, so you do them instead of your recovery week workouts, and then find that after 3 weeks of intense workouts you now have 2 more weeks of intense workouts… Then after 9 weeks you oversleep your alarm 3 days in a row because you’re completely blown to bits and end up taking 3 weeks to recovery and shove all the ice cream and donuts you can possibly fit into your mouth. Then you get back on the bike, ride Recharger… and the cycle starts all over again.