Depends on what you want.
Soft ride and a bit more rear headroom, take the Regal.
Soft old-folks seats that cause backaches, take the Buick.
Tighter suspension, 16" wheels std. on the NA (unsupercharged) version, take the GP.
I bought a slightly used 02 Regal a few years back. The suspension was so soft I immediately ordered struts and 16" wheels with decent tires. The stock tires squealed and moaned, and felt like their G limit was about .50.
I later put on the GMPP anti roll bars, which helped enormously.
I personally would go with the non-supercharged versions, given equal condition. A Buick is less likely, generally, to have been hot rodded by a cigarette-smoking mullet, than an older GP. The SC version goes double.
I've seen the occasional very clean looking Regal GS, look like they're driven by the original owner and they look like they may have never seen WFO throttle.
Given equal condition, today I'd buy the GP.
For my wants, it would need less suspension work to make it handle like something better than a...Buick.
There's a reason the car mags call it "Buick-like handling."
Plus I like the red instrument lights.
It comes down to: The car's condition; personal tastes; accepting my incredible wisdom and taste at face value.