While I've no doubt these are an improvement over tired rubber bushes not all polyurethane is the same, also the process for making the bushes can differ widely.
You mentioned the price of Superpro bushes but this is like comparing a Falken tyre to a Michelin they are not the same quality, Superpro offer a lifetime warranty for a reason
Whiteline bushes use a different material to Superpro called synthetic elastomer this is cheaper to produce hence why they are cheaper than the Supepro but still offer's good compliance on the road.
We have tried all the main poly bushes in various road cars, track cars and race/rally cars and the Superpro comes out on top, the Whiteline bushes are still excellent though and great if you are looking to save a bit of money.