I would stick with the OEM fuel pump as it will deliver guaranteed performance (in theory). Some of the aftermarket ones are fine but if it was me I'd stick with the OEM part.99 528i E39 with 150K miles.
I am getting ready to replace the expansion tank and the valve cover gasket and have recently noticed the smell of burning rubber. Am wondering if this might be the serpentine belt and since it looks OK if there might be a component that isn't turning freely that may be creating friction? The last time I had this experience on a 99 MB C280 it turned out to be the harmonic balancer. I don't want to let this get out of hand and be stranded somewhere.
Another question: Are the aftermarket fuel pumps noticeably less reliable than the expensive OEM part?
As for the smell, you mentioned having to replace the valve cover gasket. I know when mine was leaking I would occasionally get a burning rubber smell. After replacing the valve cover gasket the smell went away. I have a hard time seeing the serpentine belt being the cause of the smell but it wouldn't hurt to get it replaced as well.