One more very cool feature of User controls (and a major yucky of SSIs) is that they can be dynamically loaded with logic. This isn’t to say that you can’t dynamically execute the code in SSIs. This you can do as follows:
<%if var = 1 then%> <!--#include file="file1.inc" --> <%else%> <!--#include file="file2.inc" --> <%end if%>