What motivates me personally is not that I think Google Postgres will be better than RDS (which is great for us so far), but because I want to move to GCP for other reasons (pricing, better UI & cli, GKE, BigQuery) and the managed Postgres RDS was the only thing that was important enough to keep us on AWS
I just started using Google Postgres and, as of today, there is no way to easily get your PG data into BigQuery via any Google cloud service. You have to create your own CSV from your PG data, then upload that into BigQuery. I imagine (hope) this will change with additions to the Export options of PG instances.