Use the 50406, thats what H&R specifically has listed as the part number for your car, plus you should like the extra stiffness.
Originally Posted by H&R
Applications for 1991 BMW 325i
Stage Chassis / Drivetrain Notes Approx. Front Lowering Approx. Rear Lowering Part Number Sug. Retail
OE Sport Spring E30 not cabrio or ix 1.0 1.0 50404-55 $ 349.00 Kit
Sport Spring E30 not cabrio or ix 1.3 1.3 50406 $ 349.00 Kit