What are the true differences between these modes, and is there a benefit regarding 1. the longevity of the car, and 2 the environmental/ecological impact of the car, by using one mode over the others?
They just remap the accelerator aggressiveness. Eco also puts the HVAC into Eco mode so you stand the best chance of environ/eco positive impact in that mode.
Probably no difference in the longevity of the car and little or no impact. You can do exactly the same thing in Pwr mode as Eco mode by judicious application of the accelerator pedal. Try switching from Eco to Pwr and notice the difference. Than from Pwr to Normal. A bit more giddy-up in the modes, but no big difference. We like to use Pwr on the freeway when we appreciate the acceleration to squirt into a gap in traffic.
On my Gen 3, I didn't notice any difference in fuel economy between eco and normal modes, so I just leave it in normal mode, and that's the way it is on my Prime. However, my wife inherited the Gen 3 and she gets less gas mileage than I did, so I put her car in eco mode.
Toyota adds stuff, and sometimes deletes stuff. I'd speculate the 3 modes are something that might get deleted in the next Prius gen.
I have chronic pain in my ankle, so the PWR mode’s easy effort is something I appreciate. But otherwise, I’ve not seen significant differences in efficiency. Winter tires (just installed), on the other hand, certainly have an effect.
I was also curious about this in the beginning. It is claimed that power mode will also affect other things than the throttle, but I can't really tell. I have noticed the following in my year of driving Eco mode is useful for tiptoeing on icy surfaces Power mode is friendlier to the foot when accelerating or driving at high speeds In addition, due to how the throttle curves are laid out, I find that in Eco mode I *accidentally floor it too often. I rarely want 100%, I want 80% or 90%. Using Power (and to some degree Normal) I think I'm gentler on the engine as it is easier to stay away from the very maximum. In my EU car this also helps keep the ICE off.
Note that the eco mode also switches the A/C into the eco mode; so, you're definitely improving the fuel economy even if your foot is extraordinarily talented to drive as if in the eco mode while in the normal or power modes.
I'm in ECO about 99% of the time. Makes it easier at low speeds to fine tune your throttle opening to your actual power need. Out on the interstate, it probably doesn't matter.
It's purported, that between 3rd and 4th gens, the modes were shifted "one", towards more aggressive. In other words: "normal" 3rd gen is roughly equivalent to "ECO" 4th gen. I can attest 3rd gen ECO is absurdly sluggish; I found it hard to modulate; similar to what @Henrik Helmers mentioned.
First 3 years I stayed mostly in Normal, especially in HV mode. Some Eco, but not too often. This last year I've been using Power mode more. I'm now comfortable using any mode, depending on what I want from a trip. For me it's really just a signal to the car as to what I'll trying to do on a trip. Sometimes the car cooperates and produces the desired results and sometimes the car or my signal to the car is way off the mark. Not really too much of a noticeable difference between the 3 modes for me anymore after having used them all and comparing them on different type trips. First year I used Power mode in the unplowed snow. It's was a big difference from Eco mode the previous 4 mouths of ownership, that was before I switched to Normal mode almost exclusively for a couple of years. Just depends I guess.