The traditional green dyed coolant and the newer Dex-Cool are both formulated using an ethylene glycol base. Aside from the dye color, the difference is really more about the corrosion inhibitors. Dex-Cool uses carboxylate corrosion inhibitors and green dyed coolant uses silicate corrosion inhibitors. I don't know about molecule size for the corrosion inhibitors, but ethylene glycol is ethylene glycol and water is water (in these 50/50 mix ratios).
Aside from metal(s) deterioration related to coolant, gasket deterioration is also something to consider. I saw a lot of it in the late 1980s through the 1990s with the 6.2/6.5 diesels that were equipped with the green coolant. Head gaskets for the 6.2/6.5 have definitely improved through the years and for a variety of reasons, but it's the water pump gaskets and other wet gaskets that I saw deteriorate (that had the green coolant) back in the day. I just don't see it with Dex-Cool.
Jim