Simply put, '97 was a complete redesign. They took a riviera body, made some tweaks, threw a couple more doors on it, and the '97 Park Avenue is what emerged. Before that, the Park Avenue was a stretched LeSabre.
It depends on what you want. '91-'96 are generally more accessible when it comes to working on them, are nice and cushy to drive, but are generally more floppy when it comes to cornering and braking. '97 on is more refined, a little better engineered from a safety standpoint, but doesn't look as good in my opinion. Read up on the standard GM issues for these cars (LIM/UIM issues, DexCool, etc) and you should be pretty well prepared when looking at potential candidates.