I drove a '98 Riviera in the winter of '06, and thoroughly enjoyed it. The seats were brilliant, far better IMO than the ones in the '99 ETC I drove shortly before I drove this one. And I do remember the Riviera being more spacious inside, and even though the ETC makes more power, I prefer the torque curve in the Riviera, at least for normal driving.
But the ETC holds the advantage in terms of quality of materials, out right power and acceleration, interior and exterior design, cache.
Oh, here, I found the review I wrote up of the '97 Riviera.
http://www.cadillacforums.com/forums...7-riviera.html (Test drove a '97 Riviera)