Oil-drop Experiment by Milikan: Which technique is better?

In my opinion (1) is the better technique. It avoids having to know the exact viscosity of the air, questions about the Reynolds number of the flow etc - all the things that make drag such a difficult subject.

Further, you get "infinitely long time" to take a good measurement - you can iteratively adjust your voltage until the drops float "just so", and since the drops end up stationary you can measure their size very accurately (for example, by using diffreaction with a laser ... something Millikan didn't have the luxury to have).

Whenever you do an experimental determination of a quantity it is good to consider the sources of error, and how they propagate. In the case of the stationary drop, the quantities that go into your final answer are:

• mass of droplet (->size, density)
• electric field (->voltage, plate separation)
• local gravitational field

If you have a falling drop, you additionally worry about:

• drag (-> density of air, temperature of air, composition of air e.g. humidity)
• changes in electric field with position between plates

Note - in Millikan's original experiment, he did use the fall velocity (at E=0) as a means of determining the radius of the drop; but he used the voltage needed to create a stationary (or slowly rising) drop to estimate the charge of the electron.

In general, "zero" is a good thing to measure - it usually requires less calibration.