The general rule should be that if the ToolsVersion attribute is 3.5, then MSBuild will use the 3.5 toolset (including CSC 3.0). If the ToolsVersion attribute is 2.0, then it'll use CSC 2.0.
The general rule should be that if the ToolsVersion attribute is 3.5, then MSBuild will use the 3.5 toolset (including CSC 3.0). If the ToolsVersion attribute is 2.0, then it'll use CSC 2.0.