You should drive them both. No doubt a tune will change the characteristics. But as is the 550 to the 535 and the 535 to the 528 the displacement of the engine does impact the driving characteristics. I had the opportunity to drive the 328 for almost a week while my car was getting serviced. Here are my perspectives, recognizing that the car is smaller than the 535.
It's a good engine. The best way to describe it, is that when the turbo is spooled up you get good acceleration and the car does accelerate very well in sport mode. What you notice as compared to a 535 (again, I'm surmising) is that in every day driving, particularly in non-sport mode, the 4 cylinder engine is quite noticeable from a sound and torque perspective. By that I mean it will be very apparent to you that it is economically oriented, the turbo doe not impact the engine as much and you will find the car accelerates leisurely. I found myself constantly stabbing the gas peddle to get the turbo to spool. Compared to a 535 (and I have a Burger Tune) you will get far better every day acceleration without mashing the peddle, the sound is very different and when you really step on it, well...it goes. No different again than the step up to the 550.
Best to drive each for at least 20 minutes and see for yourself. I wouldn't necessarily rule it out, just depends how you will react to it. What I found was, I couldn't experience and significant improvement in gas mileage as I was flooring it a lot. This may be a different result for you.
2012 535xi M-Sport, Carbon Black, Cinnamon-Alum Hex, Exec Pack, Tech Pack, HUD, Damper Control, BMW Apps, Heated Rear Seats