How many rows of letter size files would fit front to back configuration on the 42" and the 36". It appears that you would 3 row for both sizes so there would be not advantage to getting the 42" unless doing legal size. Is this a good assumption.
You will be able to fit 3 rows front to back and have approximately 6 inches left over in the 42 inch. The advantage is the lateral file cabinets are made for filing left to right, where you are able to use the whole cabinet.