Carlos says the North side of the island would be best, somewhere near the coast but not too near (tidal surges due to hurricanes).
He says the mountains are shrouded in clouds most of the time but midway between the heights and the beaches would be good.
He adds that if you would be working in San Juan you will have to work out the drive time rather than the distance, as some of the roads are clogged with traffic during rush hour ... bummer.
His brother Ricky work on the east side of San Juan and lives on the eastern part of the island. He says his drive time is between 1.5 and 2 hours each way.
Carlos says the southern side of the island is hot and dry all the time but he doesn't know about the skies from there. His brother's friend with the ETX-125 recently joined the Puerto Rico Astronomical Society and he'll ask how that is going and where they observe, if they have a club site.