Pros and cons of a particular DB design approach for single vs multiple tables
Note, that I assume C# (since this language is present in tags) and Entity Framework.  

Both approaches assumes, that entity types are inherited from abstract, non-persistent base entity type, which accumulates 75% of fields, so the code base discussion is not relevant.

Pros of Table-Per-Hierarchy (all descendants are stored in single table):


database structure is simpler (just one table)
queries are simpler (with Table-Per-Type you need joins to select descendant's row)
auditing is simpler


Cons of Table-Per-Hierarchy:


the table has low-density data (a number of nullable fields, filled dependent on row type). This isn't a problem, when there are 5-6 such fields, but what if there are 20-25 of them?
table relations and indexes structure are not obvious. Since your entities will have relations with other entities, this will lead you to foreign key madness. Just imagine foreign keys for you table.
database structure isn't obvious. Who can guess, that news are stored beside CMS pages?
usually, 90% of entity types have an Id and Name-like fields (Title, Header, etc). Are you going to store all of them, using single table?
table size will grow very fast. With the lapse of time some operations (e.g., deletion) will be slower.


IMHO, every inheritance scheme, mapped to a relational database, should be used very carefully. Definitely you shouldn't map all you entities to a single table.