The Walkinshaw is made by HM, the only difference is the fancy machined tips. Proves marketing works. Pay the $1000 premium for the Walkinshaw name to get a HM/Perry system.....
Every brand has a different design saying that theirs works best for flow and power. There are legitimate differences between brands, and you would probably pick it on a dyno, but in real world driving, the only difference is note. And then, it will be an individual thing, everyone has a different idea on what sounds better.
It really perturbs me when people say that this system or that system sounds the best and that is the system people should buy. I say that people should listen to a few different systems and pick the one that they like. I push Manta because, yes, I believe they sound really good. I can also do them cheap compared to their retail price so a lot of people are buying them. When they are installed, I would go out on a limb and say that everyone has been very happy with the quality and the sound of the Manta systems. They are not a cheap imported or Ebay exhaust. (As a matter of fact, there is one shop here in WA that is selling Manta on Ebay, but I still do them cheaper
) They are one of the better systems available, sure, they don't look all pretty and shiny, but performace, note and fit are not judged by how it looks.
The only thing I will say about which system people should buy is that get the one that they can afford and that they like. Do not be a sheep and get a particular one because everyone says they should. (Unless of course they say Manta...). If you are a brand junkie and like to spruke on about what you have fitted to your car, then get the one that has had the most publicity and is the most expensive. If a cheap imported exhaust is all you can afford, then get it. But if you can stretch a few hundred extra, go the Australian made one as it has been designed, tested and refined to suit the car, not just copied and made to look all nice and pretty.