This is the best information that I've read on this forum:
traveling within dr - DR1 Dominican Republic Forums
I will do option 1 and take the Expresso Bavaro, we'll just have to be there early enough to secure seats. DR 250 pesos (assuming this is still current) is about $7 or $8 depending on exchange rate, so this is quite affordable and the buses seem quite comfortable. We'll be in SD for a couple of days before we head to PC, so we'll be checking it all out first.