Been in Gran canaria regularly over the years at different times of the year. Generally September is very good, almost like summer months but not as hot. The weather tapers off to a bottom point around xmass, but even then you can get temps of low 20's. Even if high 20's in sept (which it might be), being on the atlantic there is virtually no humidity, so its very nice outside of peak hours (12 -3), cool evenings and no mosquitoes as its basically a volcanic rock!
Porto Rico is a man made resort, most people like it a lot, but its not my preference. Plenty of eateries and bars etc and nice beach. 40mins bus ride to Playa des Ingles if you want some diversion.
Enjoy.