If you look at the two side by side the park and the union ones are identical besides the clamp. The clamp is the most important park of the stand to be honest so I'd imagine the park one to be much more solid.
Just a comparison between Park and the cheap ripoff brands.
I've used two stands that are identical in every aspect, even the clamps are the same design and even though we added a second head to the park one as opposed to buying a two headed stand straight out like we did with the cheaper stand (workshop stand bolted into floor hence double headed.) The Park one feels miles better and the only thing that has needed to be replaced is the grips on the clamp, whereas the cheaper one's clamp for the rotation of the bike has started to fail after a year, not to mention the clamp seems clunky.
But then again, you are paying for the park brand name in some instances I'd imagine.