Biggest reason to me is the BOSE 4.0 425 watt sound system. It is not available on the Deville line. The STS is smaller, nimbler, and more sporty. On the other hand, the DTS is larger, more interior room (the back seat in the STS is very small), and you can get Nightvision.
Not better! Just different, I really like both and have been thinking about getting a new Cadillac. The main problem I'm having is I like both cars equally. It just boils down to one thing.....Too Many Cadillacs, Not enough time!
I dont know what you should get, the choice is so hard. But if you plan on hauling people, they will thank you for the DTS. If you dont care about them , then go for the STS. I would personally go for that, since they are considered by most the "top of the line model"
Strangely enough, if you look at the specs, the two cars are a lot closer in interior dimensions than you might think. The only big difference is in the rear leg room. There are also a couple of dimensions where the Seville is actually bigger than the Deville!
If you compare dimensions of the two cars over the years, you'll see that the Seville got bigger while the Deville got smaller. Strange huh?
That is the one thing I don't like about my STS, there's not much rear seat room! There's plenty of room up front though. But since I'm usually the only one in the car, or one passenger, it doesn't matter too much.
I do like the roominess of my 94 Concours! But it pales in comparison to my old 76 Fleetwood!