confused, what type of water pump?

[puddinboo]

I am replacing my thermostat and while I was there I was thinking about replacing water pump and been checking online and there are metal or plastic type pumps, metal seems like it would be stronger, but the plastic pump would be lighter , and also you can replace the water pump pulley with aluminum, which I thought would be a good idea, but then I`m thinking of weight issues ,if it would make a difference or not since BMW puts plastic ones in there system,just wondering if there is a reason for this? just curious.