Back when I was shopping I drove both a 2008 Lux III and Performance back to back on the same expressway, on-off ramps and back roads.
The first difference that I noticed between the two was the tip in, or sensitivity, of the accelerator pedal. The Performance has a very touchy gas pedal from a dead stop. I found it very hard to get the Performance to launch as though I was driving with passengers in the car or like driving a limousine or hearse. Perhaps after more than an hour of seat time I would get better at it, but I really didn't like this. It's launch was like a BMW's in the sense that a very small movement of the pedal - say 1/4 inch - generated maybe a 20% opening in the throttle. This makes the car feel lighter and more powerful, but this almost completely blew me off of the Performance.
I also really wanted to like the Magnetic Ride Control, and it was very good. The car never felt like it was gliding over the road. It always felt very glued to the road, especially going around clover-leaf on and off ramps. Very confidence inspiring. However the 18 inch tires generate more road noise than the 17s on the Lux III. As well, the overall ride was noticeably stiffer than the Lux III. It was not bad at all, closer to that of a big Mercedes-Benz, but it was no where near what you may know as a traditional Cadillac ride. Although on perfect roads, the ride difference would have likely been imperceptible.
The Lux III rode quieter, with less noise and bump intrusion to the seat of my pants. There were times when at highway speed on my bumpy road course it felt like the car had sort of lifted off of the road. This occasional glide or magic carpet ride did not inspire my confidence in the same way that the MRC and Performance did.
If you like to be the first person away from a red light, if you still do a little race with the car in the next lane every couple of days or so, and if you are not overly concerned about the absolute comfort of your passengers - you are not chaufering your elderly family members - then a Performance is likely the car for you. Otherwise look to the Lux III.
But most importantly, you have to drive them back to back. I drove five hours to be able to do the comparison and I'm glad that I did. If you drove them blindfolded, I swear that you would think that they were two separate vehicles.