I owned a 1990 Cadillac Sedan deVille, and I know several other people who owned Cadillacs from the same time period. It was an excellent car--my favorite one I've ever owned--but it needed a lot of maintenence, and parts weren't cheap. Don't get me wrong, it was a reliable car in the sense that it wouldn't leave me stranded somewhere, but it always needed something or another done to it. If that specific issue doesn't bother you too much, buy a 1991-1993 model; they have the larger 4.9 litre engine which offers more power at basically the same fuel economy. Avoid 1994 and newer Devilles, as they gained a lot of weight for no improvement in interior space (sales plummeted like a rock, too). Sevilles started becomming decent around 1994 or so.
I don't know much about the Mercedes from that timeframe. At 15 years old, I figure they'll probably have maintenence issues of their own, and being a Mercedes means parts aren't going to be cheap. The Lexus was basically a cost-reduced Mercedes clone. If you're buying a 15-year-old car, you might as well buy the real thing. The Acura is much the same.
Do not buy an early 90's Lincoln Continental (it wasn't on your list, but it's also a luxury car from that time frame). Their engines were plagued by head gasket failure.
Also not on your list were a couple pretty decent cars. The mid 90's Chevy Impala SS and Buick Roadmaster (1994 to 1996) have the LT1 engine if you want something with a bit more pep, and are built on a traditional frame which makes most maintenence work easy.
If you're staying strictly to what you posted in your list, I'd say go with the Cadillac or the Mercedes. The Mercedes should have stiffer handling, while Cadillac will give you a more traditional luxury car feel. They'll both like to be pampered.
J_A_B