Hi
All these Broadcom based modem/routers do not differ very much, they all follow Broadcom's reference design and run the Broadcom software stack which the manufacturer, or usually the OEM they've contracted, adds a WebUI in the branding as required. Unless they have messed up with the circuit design the VDSL and router performance will be very similar to the next one using the same chipset. I've even crossed flashed firmwares before between totally different makes of router using the same chipset and they've worked fine because essentially they are identical.
Netgear have a reputation for not always getting the design right and have been known to release the same model in a 'Version 2' and then dropping all support on the Version 1 due to some unfixable problem.
My recommendation, go for the ZyXEL VMG8924, same specs and internals as the Netgear D6400 plus the option of VoIP built in (you don't have to use it of course), for what looks like £30 less, in much the same form factor. The ZyXEL firmware has had a few rough edges but I'd expect the same on a new release of a Netgear model, just the way we are sold consumer devices like this these days, and quite a few of us here are finding the VDSL connection rock solid on the ZyXEL.
Regards
Phil