We are a geriatric practice that provides primary care in a clinic, nursing home, skilled nursing facility, and (soon) a hospital. What RVU component(s) would be the most appropriate to use to measure physician assistant productivity?

Most practices just use the Work RVU component. The important thing, though, is to make sure you measure everyone the same way and compare yourself accurately to external benchmarks. That is, don't compare your work RVUs to an industry benchmark on total RVUs.