*S: +2-3 Econ Expansion, -1 Tech
*M: -2 Econ, 1 Forest Slot Consumed, -1 Tech, +6 Econ Expansion, other effects [King]
So two secondaries gives 4-6 EE and -2 tech, both refunded, main gives 6 EE, 2 more ee at the cost of econ, and -1 tech (refunded), an extra forest, and (Other Effects)
Since a forest costs 2 mains, that's four secondaries, is better, but it would have to take place over four turns because two secondaries combine to one main. The forests version also gives interesting effects in both Black Soil and Forests, and the forest can be doubled with Charcoal Kilns, and having more forests is always better.
So in terms of raw EE and LTE, four secondaries gives 8-12, all tech refunded, and the main gives 7(6 from black soil, 1 from forests), however, the secondaries have to be done over four turns (or two and two PSNs, reacts, etc), and the main pair gives a whole lot of (Other effects) and can be done in one turn.