The weight difference may literally be the equivalent of a bowling ball on each corner but where the weight is makes a world of difference. This is unsprung weight and the effect is dramatic on how the car rides, handles, and accelerates.
The heavier the unsprung weight the slower a wheel assembly is to rebound from a sudden change in surface. It's slower to respond to acceleration from the drive train also. These aren't simply mathematical differences, they're noticeable. Do a little research on the effects of added unsprung weight.
17" BMW sport wheels come in at around 18# per wheel, some of the lightest BMW wheels out there. If you're driving a 325 I would pay attention to wheel weight if you're trying to get some sportiness out of the car. If all you're after is what you think looks cool then it doesn't really matter. Up to you.
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