I am planning a new application and have been experimenting with GWT as a possible frontend. The design question I am facing is this.
Should I use Option A: GWT-RPC
We ran into the same issue when we created the Spiffy UI Framework. We chose REST and I would never go back. I'd even say GWT-RPC is a GWT Anti-pattern.
REST is a good idea even if you never intend to expose your REST endpoints. Creating a REST API will make your UI faster, your API better, and your entire application more maintainable.