The B+W filter is glass, like the excellent ND grads from B+W's parent company, Schneider Optics. The specs of the filter you link don't say, but the Schneider filters are 4mm thick, vs. the 2mm thickness of the resin filters from Lee, Singh-Ray, etc. If you go the glass filter route, you'll need the Lee 4mm side guides (for which reviews are mixed), not sure if you can use more than one filter at a time.
I found this link
Which says it is 2mm.
So the question is would it be better to get three Lee resin filters, or one glass B&W filter for a bit more? I'm inclined to just get the B&W.