One of my friends has a 1998 Olds Aurora (Pearl white QQX with chromies), and his brother owns a 1999 300M. They both agree that the Aurora has the smoother ride, but handles just as well as the 300M. They agree that if they had the choice to buy either one knowing what they know now, it would be a coin toss. They are both great cars, but if you want the high end power the Rora has more. And there isn't much for aftermarket perf mods for the 3.5 DOHC Chrysler, not that the N* has much but there is more.
As for the Riv, they don't ride quite as nice as the Rora, but they are still a damn tight car.
The Regal GS/GSE is/can be a fast ride and look damn sexy at the same time. Too bad I didn't buy that one I was looking at last year. Firemist red (not many made in this color) with the factory chromies and that thing hauled. But be fore-warned that you'll be sacrificing ALOT of ride quality by going to both this or a DSM.
BTW, what do you think you'll be wanting for the car (my brother is looking into selling his 1988 Olds 98 Touring Sedan with almost 300,000 miles and getting a Rora -he'* looked at a few, but they were broke, beat, or the seller'* were full of BS).
Also, Mikey, do you have anything as far as subs or amps go that you'd be selling (I'm looking to spend $200 max for a good used amp and sub -I'm cheap)