Single Table Inheritance VS Polymorphic Associations

I am trying to evaluate the reuse of software between versions.

With out going into how I'm doing this i have a style question:

If I have model Artifact, which right now can be typed as SrcCode or Document and basically is a file on the system right now I'm doing this by polymorphic associations which works well for the Artifact class

I need to create ReusedArtifact(s) which is a composite of two artifacts, and illustrates the differences between the two artifacts. The problem is in order to use polymorphic associations I need to save the base object (ReusedArtifact) before I can create and use it's sub class (ReusedSourceCodeArtifact or ReusedDocumentation) methods....

We have these fields for the two types: DOC: (words) wdiff_added wdiff_common wdiff_changed wdiff_delete SRC: (lines) diff_added diff_common diff_changed diff_delete

Personally I think that a single table inheritance setup, with added,common,changed and deleted as the fields would work well. On the other hand can anyone forsee a design problem that could be caused by counting and storing two different things (words and lines) in the same fields...

Thanks JEsse