Unfortunately, I don't have the funding to experiment with dropping flashes from heights onto hard surfaces. However, I will make a few surface-level comments. All of these flashes are made of plastic, and even the Pentax-brand modules seem like it, especially when put next to a solidly-built camera like the K20D. The Metz flashes subjectively feel the most solid to me, but this may very well be mostly because of the type of plastic used for the outer casing. The Sigma and Promaster flashes seem a little more flimsy than the Pentax or Metz ones (especially when moving the tilt/swivel heads), but not so as they'll fall right apart or anything.
The new Metz 50 AF-1 and 58 AF-2 models use metal for the flash foot; all others are plastic. Plastic is not necessarily bad, as it's better for the flash shoe to break than the camera's hot shoe, but obviously metal is more durable.
There have been some complaints about the Pentax flashes getting their zoom reflectors stuck and needing to be sent in for service. Research on the web indicates that this is unfortunately common with zoom reflector flashes of all brands (including those from other camera manufacturers), and I don't think Pentax is particularly bad in this regard.
There are also several reports of the hot shoe locking pin getting stuck with Pentax flashes. Sigma and Metz also use this locking pin, and I've seen no equivalent complaints, but it may just be due to a smaller sample size. I certainly found the Pentax locking lever the easiest to use. Metz's more solid feel also carries through here, with the Sigma and Promaster units feeling a bit more klunky to get on to the shoe.
The Tumax flashes are cheaply made and feel like it. Fortunately, they are also cheaply priced. If you're looking to save money, though, note that I've seen many anecdotal reports of these flashes failing within days, so make sure to buy from a reputable dealer.