My wife received a replacement OG EVO for her EVO a month and a half ago, even though they were giving out EVO 3D's for a long time for OG EVO's. Her best friend had received an EVO 3D from Asurion last December in place of an OG EVO.
So even if they have started given out GNex's as replacements to Nexus S models, you still run the risk of getting a Nexus S. It all depends on what is in stock when you call. They constantly get old devices that are rebuilt from the OEM, even after EOL. You are rolling the dice, no matter what.
Robert