The Seville and Pontiac Grand Prixes are both nice cars. One's obviously about 30% more expensive, but you probably get that back in interior refinement and overall quality. The performance between the two is so close, you won't be able to tell a difference in the field. I drag raced a '98 or so 4-door GTP from a stoplight one time. Neither one of us won -- it was a dead-even match. Had we ran up through illegal speeds (rather than stopping at 50 mph), the Seville's Northstar would have proved victorious.
It's an absolute joy to drive 80+ mph down the Interstate in. It's quiet, fast, and pretty discrete. Lots of rice boys will want to play with you. One application of the go-pedal will send them scurrying home.
The reliability should be good. Both the engine and transmission are strong. I've gotten pretty good reliability from mine. The only major thing the engine has needed has been an alternator and water pump. I put in a new thermostat when I changed the water pump this weekend. Aside from that, everything has been very good for me. All the structs/shocks on the car are still original and in good shape. The ride is very smooth and enjoyable. I really don't have a single complaint about the car.
I think you'll notice much more refinement in a Seville vs. a Grand Prix. The 98+ Seville is built on the G platforum (Aurora) and is one of the stiffest structures GM makes. If you get one, go for the Bose 4.0 system. It's supposed to make Carnegie Hall sound like AM radio.
The 32-valve Northstar will sound and feel much smoother than the pushrod V6 in the Pontiac. It's true that the aftermarket will give you more bang for the buck with the Pontiac, but I suspect that's not necessarily what you're after. The Cadillac is the underspoken but powerful guy at the party -- and the Pontiac is the track runner who just has to let everyone know how fast he is. Both will get the job done, but both have very different styles.
I like the style of the Caddy. You don't have to put on wheels and exhaust pipes to let everyone know how fast you are. You've got it at the beckoning of your right foot if you need it.