I went with Bilsteins HD (B6) and converted to coil springs in rear and loved it. Sold it after few years and I'm in touch with new owner who loves it too.
https://xoutpost.com/bmw-sav-forums/...gs-delete.html
I've actually got identical parts on shelf and will do refresh on my current X5.
But no conversion required since my X5 is Sport package and came with coil springs in the rear.