Erg mode: TrainerRoad will set your trainer’s resistance so that you are putting out the required power regardless of your cadence, you just have to pedal.
Resistance mode: Power will be proportional to your rear wheel speed and it’s up to your to adjust your cadence and gearing such that you are putting out the power that TrainerRoad is asking.
The quality of the workout shouldn’t be any different (as long as you can hit the power required in resistance mode and don’t get lazy).
Recommendation is to do the new ramp test in erg mode, but you need to do the old 8 and 20 minute tests in resistance.