There's no difference between the plastic and aluminum housings; except that the plastic ones can crack with age at the openings where your upper & lower radiator hoses attach. That's the only reason why aluminum is better.
Larry, the shop may have done the procedure incorrectly if you keep leaking. First, the thermostat has an arrow on the edge that should point upwards when installed. Make sure you lube the thermostat & housing gaskets with coolant, don't just stick them in dry. Lastly, the bolts should be tightened to the proper torque, no more, no less, or the gasket will not seat or seal properly. Engine lift bolt (the big one) should be 17 ft-lb, and the remaining bolts should be 89 in-lbs.
As for the coolant, you really should be using the blue BMW stuff if you want the motor to last. The green stuff contains phosphates which cause pitting and corrosion on the alloy engine block over time. There are some brands that have similar properties as BMW's coolant, but I've yet to see a general consensus as to whether they really are safe to use.
Hope this helps and good luck!