There are tons of discussion about this shock... Do a search or bmwe34.net. It boils down to this...
OEM... Boge
OEM plus... Sach
Firmer... Bilstein
$$$... Koni
Which is more reliable? Probably the Bilstein. Boge and Sach will wear out... Koni has its own problem with leaking. Bilstein as some reported to have problem... but I never did with many cars.