Here's something I posted on another forum for a similar question--
<<I am a current STS owner and I would never buy anything else. It is the most marvelous Cadillac that I have ever owned and I've owned many. It really is a driver's car from the moment you get into the car. The seat automatically moves into position as does the steering wheel and the 2 outside mirrors, your favorite music begins where it left off, your environment is automatically controlled without you having to adjust anything, and the car even displays your name for you as an added reminder that "Best of all--It's YOUR Cadillac!" (<--I've always loved that slogan!). Storage space is abundant so you can store whatever it is that you want to store. The sound system is so incredible (BOSE 4.0 425 watts that was built specifically for the Seville) that you will not want to listen to music anywhere else. The wiper system that automatically adjusts wiper speed according to how the rain is coming down but the best part has to be the Northstar System. Not only do you have a very comfortable car to travel in but you have all the power that is inherent to the Northstar. Once you step on the accelerator, you get thrown back in the seat and you get this wicked grin on your face as the speedometer needle goes higher and higher. You get it all with the STS--comfort, sportiness, power, features, luxury and speed if you are so inclined.
With the SLS, you can get one that is comparably equipped to an STS but there are also ones out there that are "stripped." No BOSE, memory (which includes the power tilt/telescope steering wheel), Rainsense, garage opener, fog lights, heated seats (all 4 outboard seats) or 6 disc CD changer. You also lose 25 horsepower (which doesn't seem like much but you can definately tell the difference). The extra torque is almost negligible. The interior look is a lot "softer" with the printed inserts versus the perforated Nuance leather. The STS just looks more aggressive with the complete Zebrano wood going down all 4 doors, the console, shifter handle and steering wheel, the sportier wheels, the exhaust tips, the rear smoked amber lenses, the fog lights and the darkened wreath and crest on the grill.
Hope I've helped!>>
The difference between the D'Elegance and the Concours is greater than between the STS and SLS. The engine differences would be the same but the ride and interior to me are world's apart. My favorite riding car would be the 97-99 D'Elegance (or Deville). It is one of the most comfortable cars to drive as well as to be a passenger. There is an abundance of room and you feel like your riding on a cloud. You still retain 6 passenger seating if necessary since there is no console. With the Concours, you do get the console which limits your seating to 5 and the suspension is much stiffer but it still rides very nice. Of course you get more standard features along with fog lights and a grill mounted emblem. I still like to look out the driver's window and see the shimmering gold stand-up wreath and crest which is standard on the D'Elegance to remind you that you're driving the ultimate in Domestic luxury.
In some cases there may be good reason to resurrect a thread............... and in other cases .....................................
(and, FWIW a "CAI" for a Seville is less than useless (the factory setup is already more than adequate [but not noisy]; there are still no engine modifications out there for the Northstar in a FWD Seville/Deville daily driver.)