I have the HDZ as well, really nice, top-notch quality and @Luke has been very responsive on support.
Recent review/feedback of the CNC4Newbie: Z-Axis Upgrade from CNC4Newbie.com Initial Review
A quick glaze of the TBDcnc, looks to be a little less stout. The HDZ has thicker aluminum plates (solid black plate vs the extrusion also) and larger size of linear rails. TBDcnc seems to be XCrave focused, might recommend looking at the XCrave community and see if anyone there has something to say about it.