Glenavil,
You will gain a Multi Engine Piston Class Rating (MEP Land), which covers all light piston twins, including BE-76 Duchess, Cougar, Seneca, Seminole, Cessna 337 BN Islander etc etc.
There is no specific type rating for a Seneca.
As for air taxi work, for single pilot ops you will need a minimum of 700 hours.
If I were you I would look at the school that will teach you the most in the atmosphere that you feel most comfortable for the least amount of money.