I vote for the Park Avenue Ultra.
A well maintained DeVille is a very nice car. A neglected DeVille is a horror. The Northstar powerplant requires regular cooling system maintenence to avoid catastrophic failure. Check some of the Cadillac-specific forums online and you'll see some really heartbreaking stories.
If you do settle on a DeVille, seek out a DHS. You get many more features than the Base model (sunshades, Bose, heated rear seats, etc.) while retaining the very comfortable bench/column setup. With the DTS, you're stuck with space-robbing console. Also, the DHS has the beautiful analog gauge cluster from the DTS, not the cheap looking digital cluster from the Base.
My advice: stick with the Park. It's more reliable, cheaper to fix, cheaper to run, cheaper to insure, and the Supercharged 3800 isn't going to grenade on you.
Good luck.