Normally I go for New Chemicals factories, but if you look at the math Research factories pay for themselves slightly more quickly than New Chemicals do. (Average value of 13 for $30 instead of average value of 20 for $60).
You aren't taking into account the cost of the worker. Assuming that you pay $10 for each worker/robot it is 13 for $40 or 20 for $70. It comes down to how plentiful your workers are. If you are worker tight New Chemicals are much better since they require fewer workers for the same amount of investment. If you have cheap/plentiful workers/robots then Research is better.