1999 is when they went through the internals and redid things ... so it should be basically the same as the 2000. 1999 also has a sport package simmular to the 00' and 01's.
I myself like the rear of the 97-99's but to me the front looks ugly because of the bumper protectors.
For 2000:
"Functionally, the 2000 Catera features an improved up-level BoseŽ entertainment system, new oil-to-air transmission oil cooler, revised rear damper design, improved power brake booster and recalibrated steering gear system."
Its known that they took the sport suspension from the 99 and carried it to the 2000 putting it as standard and having an even more improved sport option. The engine items carried over from 99 also and slightly improved again. Read the links for all the info...
http://media.gm.com/division/cadilla...ss_release.htm http://media.gm.com/division/cadilla.../00summary.htm
Its funny, i was comparing interior specs on another site... and the Catera (2000 specific) has a larger interior and larger trunk than the CTS it was replaced with.