Everyone has a different experience with various brand names, so I can only speak to mine. I used PUI seat covers as recommended by my installer, with 35+ years of experience. He has used both and swears by the PUI brand. He says it's all about the experience of the installer and taking the time and knowing the "tricks" of seat cover installation. I have had mine for 7 years, and they still fit tight and smooth.
The fundamental differences I see on the Legendary versus PUI comes down to two main points...
1...Thickness of material and depth of heat pleats, nothing is like the factory originals but legendary seat covers retain far more of the rolls between the heat pleats than do the PUI covers once installed, this is because the legendary covers are made of heavier grade vinyl and the heat pleats are therefore able to be made deeper. This has a huge effect on how much more correct the cover looks installed. There is no comparison here.
2...Color, as a rule (but there are exceptions) the Legendary covers are much closer to the original colors than PUI. This is most commonly an issue seen with the golds, greens and blues.
That being said that doesn't mean all is hunky dory in Legendary land, I've had no personal issues but I often hear about production delays and generally lousy service attitude about them.
All that being said if you want the most correct looking covers you have to go with Legendary if they are offered for your car (PUI has a larger offering especially in the less popular models than Legendary does).