Based on cost and availability I went with a waterpump from Advanced, we have a few of the stores near me so if mine fails I'll be able to get the replacement for free. To me there is NOTHING particularly 'robust' with our cooling systems, this is a known weak spot in the E39 line. It certainly does not make it a 'show stopper' but, as we all know the cooling system is certainly somethign all of us E39'ers are familiar with and know well. If we get 3-4 years out of a cooling system component then we consider ourselves lucky. I know there are site sponsors, and the popular sentiment here is to use OEM. Logically speaking that is all good, but from an owners and DIY'ers perspective you need to have a price/performance paypoint and there is no performance gain by using any of the 'OEM' components therefore price would be a deciding factor. When you can pay for one water pump and end up with 2 or 3 before you sell the car, it only makes sense to get one with a lifetime warranty, same with Radiator, expansion tank, hoses, clutch fan etc...
99 540i-6 Sport & 07 Z4///M Coupe