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