You do tend to get what you pay for imo.
Generally the welds & materials on cheaper pipes won't be as good as the more expensive ones. In extreme cases it could lead to the De-Cat pipes coming away from the flanges or blowing from other places although they would have to be extremely bad for either of those things to happen.
There will be differences between brands yes ~ Different stainless steel grades will be used, some may have larger diameter pipes (even though the mating flanges may be the same size), slightly different bends (whether intentional after R&D or just accidental I don't know), some have brackets to help support the exhaust system/others don't, some will have the longer 02 sensor mounts in an attempt (not always successful) to stop the EML coming on when De-Cats are fitted.
I chose Berks De-Cats when I fitted them to my Zed as at £150 odd they were hardly stupidly expensive. The quality of Berks products imo is very good, ...the welds were clean with no breaks in between and everything lined up perfectly when fitted to the car. Mine was mated to the standard Nissan OEM manifolds and my Invidia Gemini exhaust system. Sounded incredible too.
They are not legal (for off road use) but literally thousands of cars do have them fitted. You can either choose to refit standard Cats each year for your MOT or go down the route you mentioned but then what's the point of getting De-Cat pipes at all if you plan on welding Cats/HFC's further down your system?!!
Might as well just get HFC's instead of the De-Cats and save yourself the hassle (oh and I did have HFC's before which again were made by Berks which were fantastic).
If you have a friendly MOT person though then your car with De-Cats will pass the MOT but then that's a whole other kettle of fish.
Oh and do not get anything exhaust related (De-Cats/HFC's/Y-Pipe/system etc) that has those wire flexis in if you can help it at all as they are the weak point of any exhaust part/system imo.
Hope that has answered all your questions Mj86 (my old motor in below video).