Edward,
As far as I know Cubic Tech makes 3 versions of hybrid cuben. All of them are 1.43oz cuben laminated to a woven polyester face fabric. The three versions are referred to as wov20, wov32 and wov75. These codes refer to polyester face fabrics of 50D, 70D and 150D respectively (I could be slightly off on this, but it's close). The polyesters weigh 1.5oz, 1.9oz and 3.5oz, so add on 1.43oz for the cuben backing and you get 2.9, 3.3 and 4.9oz total weights.
Zpacks uses the 2.9oz version, while ULA uses the 3.3oz stuff. I think HMG was using the 3.3oz stuff, but looking at their website now it seems they are using both the 2.9oz and 4.9oz stuff. Back bottoms and heavy duty packs (i.e. Porter) use the 4.9oz stuff, while smaller packs predominately use the 2.9oz stuff.
I've got several hybrid cuben packs in the house and it's decent stuff, but in my opinion Xpac fabrics are better for packs. Hybrid cuben consists of woven polyester + mylar + spectra fibers + mylar, all glued together in a sandwich. The cuben component (mylar + spectra + mylar) looks good on a spec sheet (high tear strength) but it's not actually that functional in a pack because the spectra isn't woven so it does very little to stop stitch hole elongation and punctures. It does stop tears from expanding, but by the time you're at this point the cuben is a fraying mess anyways. The durability of a pack fabric is largely determined by it's abrasion resistance and puncture/cut resistance, so with hybrid cuben this is almost entirely coming from the polyester face fabric and then the cuben component is mostly there for waterproofing. My complaint here is that using cuben as a waterproof layer is an expensive and relatively heavy way to make a pack waterproof. A single layer of tougher plastic can be both lighter and more durable when faced with internal pack abrasion. With cuben, the inside mylar eventually starts to degrade after heavy use, and then spectra starts fraying everywhere.
Xpac fabrics use a single tougher layer of plastic (0.25mil or 0.5 mil PET) rather than cuben to waterproof the pack. The fabrics come in a wider range of woven face fabrics (30D to 1000D nylon) and can be had with or without an inner woven fabric to protect to plastic. The result is lighter, cheaper fabrics for similar functionality and the option of more durable fabrics. Consider Xpac TX07 vs 3.3oz Hybrid Cuben. Both materials use a 70D woven face fabrics but TX07 waterproofs it with 0.25 mil PET while the hybrid cuben uses 1.43oz cuben. Both materials should be similarly durable, but the TX07 is lighter (2.9oz vs 3.3oz) and the plastic waterproof layer will likely hold up better over the long haul. Plus TX07 is $16/yd instead of $39.
For a durable ultralight pack, XPac X21 (4.4oz/yd) looks great. It's 210D nylon + 0.5 mil PET (double thickness). For a pack seeing a lot of off-trail use then X33 is great (330D + 0.5 mil PET).
