Architect Jon Brouchoud, who's used Second Life for several innovative applications in his practice (such as this one), has a collection of interesting videos showing off realtime architecture visualization in Unity3D, among them this:
Many more on his blog here. Pretty impressive. I've seen better-looking architectural visualizations in Second Life and in OpenSim (such as this one), but Unity is the more open, more popular platform, and can run on the web, so it has the advantages there. I'd say SL maintains the advantage if you as the architect are already extremely familiar with the platform, and are planning to demo the SL visualization to the client yourself, as LA architect David Denton did for a major Egyptian project.