Congratulations on your engagement! Welcome to the forum.
I went through the same decision (Cabo vs. PV) and I decided on PV. It's more tropical in mind where as Cabo is more desert. However, I love both!
I have not been to the Riu in Cabo but I stayed at the Riu Jalisco last fall. I really enjoyed our stay and yes, the beach is nice. However, I'm not certain where they have the ceremonies at, but the gazebo (which might be it) is in the center of the hotel 'courtyard' and is not very private (just something to consider).
I'm sure you will be happy with either location. Best of luck with your decision. Location is definitely the hardest decision!