The main difference is that ErgVideo™ is power-profile based, and other simulations are elevation-profile based. ErgVideo™ operates in what is known as "Ergometer mode", and products like RacerMate™ RCV operates in elevation profile mode, what I'll call "3d" in this discussion, to indicate it is the same mode in which RacerMate's 3D operates. The Erg mode and 3d mode are vastly different, and I'll try to explain.
Let's first begin by recognizing that modern, successful cycling training is based on power measurement: workout sessions are often prescribed and always analysed based on the actual power output by the athlete during the session, not how far or how fast she rode. We know that many factors affect how far and how fast. For example, a rider can cover far more miles, faster, in a strong tailwind compared to a strong headwind, at the SAME physical power output. If the power output is the same, the physical development of the athlete in the session is the same, and the speed and distance covered are irrelevant to the athletic development. If you care to argue about cadence, well, make it equal too, and my point holds. Your CompuTrainer™ or VeloTron™ is a great tool with which to measure (and regulate) your sessions in terms of power.
In 3d mode, the resistance you feel is computed from the estimated real-life opposing forces you would feel when riding on a road with the currently displayed slope, or % grade. The 3d simulation computes this from your weight and the current slope (accounting for gravity and rolling resistance opposing forces), and your speed (your air resistance opposing force is computed based on estimated size and whether you have set a windspeed in your course file). The point is that on any given road or slope, you can choose to go hard, say at 300W, or easy at 100W. On the same slope, you will go faster in the first case, as in real life, but you can certainly choose to go slower at 100W. So when you ride outdoors with a power meter, ride a manual trainer with a power-meter, or when you ride in 3D mode and you want to "hold" a target power (per the goals or prescription of your training session), it is up to you to constantly watch the power level to keep on target. If you slow down, or gear down, your power goes down. If you speed up, it goes up. If you hit a hill, a higher contribution of your power must go into overcoming gravity, and if you don't slow down, your power will have to go up. Holding a "target" on the road or in 3d mode requires a great deal of concentration, gear and speed changes, and almost total focus on the power output number, rather than on the other excellent visuals presented. You will spend most of your concentration on these things, and your rides will become all about following the numbers....that alone becomes boring and more work than it should be. Any targeted ride you do in 3d mode will only be about watching the data, and regardless of the visuals, they will seem very much the same. You will be back to the boredom you had hoped to avoid by getting a CompuTrainer or power meter!
Erg mode completely turns this aspect of "needing to watch the target" on its head. The CompuTrainer actually takes-over the task of tracking power for you! If you set the CompuTrainer™ to 300W (say in stand-alone mode or Erg Mode in the Coaching Software), it will always FORCE you to do 300W, or stop! In Erg mode, the CompuTrainer is a feedback control system with power output as the set-point controlled parameter. Power is proportional to torque times speed. The torque is proportional to the magnetic force exerted against your riding by the load unit. The CompuTrainer computes your power output at every moment using the press-on force of the roller determined in the calibration phase (to compute rolling resistance contribution of your power), the current speed, and the current magnetic drag-force being exerted. It adjusts this force every 30ms. When it senses your speed go UP (seen as your attempt to increase power above set-point), the CompuTrainer will AUTOMATICALLY adjust the magnetic drag force DOWN in a manner that restores you to the target. The torque will go down so that the set-point target power (=torque x speed) is restored. Likewise when your speed goes DOWN, the load unit will INCREASE the braking force to make your power settle back to the target. You CAN pedal slower...but you have to push harder. If the power setting is above your ability to provide it, it will feel progressively harder until the CompuTrainer just stops you. The CompuTrainer is very good at making you hit the target. It only fails at very low power settings, where the rolling resistance power dissipation exceeds the setpoint power. For example, if you are going at a speed where the (calibrated) rolling resistance against the roller contributes say, 120W, and you have set the CompuTrainer to 80W, then it just turns off the magnets. It could only bring your power lower if it could relieve the press-on force or drive the wheel as with a motor. None of this is really an issue, since it's at piddly-low power levels anyway. It becomes an issue if you have applied very high press-on force, and that will affect the power level at which the CompuTrainer turns off and lets you just roll against the tight roller, uncontrolled. The Erg mode is the best way to be sure you execute your workout prescribed. You can't get lazy.
ErgVideo uses the erg mode so that you are FORCED to follow the racing and training action on the screen, and to feel the large power transitions required in a bike race, just as in a prescribed training session. Most people are initially surprised at being forced to perform. They expect the ErgVideo ride to feel like any other trainer ride where they have the ability to ignore the instructor, or take it easy whenever they feel like it. ErgVideo™ simply won't let you, unless you stop. You CAN make threshold adjustments on the fly if the workout really is over your head, so you can always finish your sessions at a lower than expected level, if you find your initial settings were overly ambitious. ErgVideo is pretty good at uncovering those who boast and exaggerate their fitness; there is no hiding.
Finally, now that you sort of get it, let's consider a flat 4-corner criterium course in 3d, vs an ErgVideo of a 4-corner crit. Now you can see in 3d there is no incentive at all to ride as if you are really in a crit, with large surges out of each corner, chasing down attacks. You will eventually just ride steadily as if riding on a flat ribbon of endless road, and you won't feel the corners and their need for deceleration or acceleration, either. In a criterium ErgVideo, the power you ride is a scaled version of the actual power output of the rider in the race. You will have to jump out of corners, and chase down the attacks, just as the cameraman had to. As well, in a crit, if you want to stay in the race, you can't so easily pick and choose when to go hard or easy, you have to follow what the race does. So here is where the "power profile-based" simulation results in a far superior simulation and event-specific workout for you.
We published a great newsletter in 2014 where we covered all of these aspects in great detail. It's here and it's worth a read.