I need to mock a Stubby server. Should I use gMock or the service mocker?

To quote PiotrKaminski, the author of the service mocker:

You can find an introduction to the service mocker here, and detailed documentation in net/rpc/testing/public/servicemocker.h. As I'm the author of the framework my opinion on it can hardly be objective, but here are the main advantages it has over gMock when it comes to mocking Stubby services:

• Services are mocked dynamically so there's no need to manually write mock service implementations.
• The client's calls go through a real Stubby channel, which will catch some errors that calling a service implementation directly would miss.
• The service mocker is aware of sync/async client distinctions and common Stubby threading strategies, and in general allows you to exert more control over when the callback is made.
• The base syntax and semantics are very similar to gMock, but Stubby-specific matchers and actions make the testing code more compact.
• A powerful expectation grouping mechanism allows expressing complicated async call ordering constraints in a readable fashion.
• By the end of the week, there'll be built-in support for testing call cancellation.

Some disadvantages:

• The service mocker documentation is not as good as gMock's.
• The error messages are probably not as good as gMock's either.
• You can only mock services, not arbitrary classes. Expectations do not interact with gMock's.
• Slightly different expectation matching semantics in corner cases, which could get confusing if you're using gMock as well.

In my biased opinion, if you only need to mock out Stubby services, you should look at the service mocker first. If you need to mock out other classes too, and especially if you need to express relationships between service and other expectations, you're probably better off with gMock.