The 2mm is what we have been calling 1 start. The T8 4 start has 2mm pitch, but 4 starts, so it move 8mm in one turn.
The difference and tradeoff is in speed. The 4 start (8mm) is faster. So the motor moves in an “ideal” rpms for most operations. Stepper motors lose torque at high speeds (that penalty is reduced with higher voltage supplies, but it it often misunderstood).
The 1 start (2mm) has more force at slow speeds, but moving at 10mm/s (the default max speed in the firmware), with the standard 12V PSU causes many builds problems. The steppers skip steps because the friction is the same in these leadscrews, and isn’t affected by the extra force. When the steppers move faster, they lose torque and start skipping steps. The solutions are to either go to a 4 start, reduce the top speed, increase the psu voltage, or some combination.
We know 12V, 4-start, 10mm/s works. Outside of that, we have some guesses, but we don’t have 10k users that have tested it.