If the manufacturer is a reputed company with good quality control, then there is no difference between the quality of equivalent products. For example the retail price of Amlodac brand (Cadila Healthcare) of amlodipine 10mg is Rs. 14.41 while the Amlopres brand (Cipla) costs Rs. 37.45. Both are equivalent manufacturers. Reason for such differences is the profit margin of the two manufacturers.
The shelf life of different medicines is different. One should not buy medicines whose expiry is near. In the case of B-Complex, the efficacy will be marginally reduced but the more important issue is the way a medicine is stocked. If kept in hot or humid weather, the decline in efficacy is quicker.