Test RPC is a client for testing:
testrpc is a Node.js based Ethereum client for testing and development. It uses ethereumjs to simulate full client behavior and make developing Ethereum applications much faster. It also includes all popular RPC functions and features (like events) and can be run deterministically to make development a breeze.
While Geth is a full client in GO Language that you can use to connect to the real chain or start your own testnet server.
geth is the command line interface for running a full ethereum node implemented in Go. It is the main deliverable of the Frontier Release.
Of course, some succeed tests in testRPC cases can be transfered to geth.