hold the plant with one hand and pull the beans off with the other.

I know how to do it, but I want to know which way is better for the plant.

Provided they were done in such a way as to avoid damaging the plant, you could use any of those methods. Personally, I pluck the beans from the plant one-handed, by grasping the bean in my palm, and pulling the stem off with my thumb & forefinger. With some practice, this can be done very quickly, without damaging the vine or other beans on the stalk. Snapping or two-handed plucking would be just as effective, but regardless of the method used, care should be taken to avoid pulling or damaging the stalk.

You can use a scissors, but I prefer not to, for two reasons. The first is that it is time consuming... which may not matter if the crop is small, but I can't imagine picking a 30-40' row of bush beans that way.

The other reason I avoid using scissors - going back to the original question - is just good garden hygiene. Any cutting implement can transmit disease from plant to plant, so I try to use them as little as possible. I'm admittedly over-cautious in that regard... but if you don't harvest from diseased plants, there should be no harm.

I only have a few plants so disease shouldn't be a problem, but I probably shouldn't dull my scissors on something indifferent.