This is no small question. Both systems have their advantages. I guess, the key to your question is why are companies migrating Oracle data marts to Teradata...
Multiple data marts have inherent relational integrity problems, it can be a nightmare or impossible to equate a given row/column from one system to another. Not just the specific data but the different qualifications used on each system for each column value. This is one of the key issues that make a Data Warehouse so attractive.
Teradata can support much larger volumes of data than Oracle, this can be demonstrated by a quick review of reference sites from both companies asking for a list of production systems of 10TB or larger and actually calling them.
Teradata has proven scaleability, so even if you start small you can add capacity as your requirements grow.
