There are a bunch of differences, the first being that RC-Physical employs legal placement under the hood. And this includes its incremental placement, which is crucial as you do optimizations in synthesis based on the placement. And the incremental legal placement also enables you to hand off the placement at the end of synthesis, seeding the back-end flow with exactly the view of the design that you saw out of synthesis.
The other major difference is that RC-Physical has full capabilities for congestion analysis, fixing, and prevention. For more information on that, check out the archived webinar we recently did: www.cadence.com/.../event.aspx
What is the major difference between RCP and RC-Spatial?