92-97- STS and ETC are going to be the same, since they are the same weight, same size (ok, the STS is 3in longer), and same suspension. Not sure on 98-02, but i am fairly sure that the ETC used the same suspension as it did in earlier years.
In 1998, the Seville moved from the EK-chassis to the G-chassis that the Aurora is built on. It's supposed to be the stiffest chassis General Motors has (or was at the time). Supposed to do many good things for NVH, handling, etc. They also came out with CVRSS, version 2, sometime in the late 1990s, so it's hard to compare apples to apples if you're looking at two different suspension systems and softwares.
The Eldorado continued on the EK platform until its demise last year. I don't know if they upgraded to version 2 on that car as well. ??
If you compare a 2002 STS and a 2002 ETC, they'll be two very different cars I'm sure. Comparing a 1997 STS and 1997 ETC should reveal very few differences, if any, in seat-of-the-pants performance, ride, and handling.
Unfortunately, I wont be getting an '03 STS. I'm not having the kind of year to spend that much money... The '01 at about $30k is all I can do... I wont bother asking for a test drive - that's one complaint I have, I guess, about dealers.. They're pains in the asses when it comes to that...