I am by far no expert as I have only lived in PR for almost 3 years but I would definitely suggest going with short term only as you mentioned. From my understanding, it is very difficult to get non paying, long term renters out of a property here in PR, particularly if they have children.
Any of the more popular tourist areas would be best. San Juan & immediate surrounding areas as well as Rincon would be good choices but I would think that Rincon might be more seasonal (winter being higher season catering to surfers)
We were looking at the possibility of purchasing investment property for short term rental and we were focusing on the San Juan area. After hurricane Maria, we wanted to let the dust settle to see how things in the rental market shake out
Hope something here might help. Good Luck!
Brian