I would have to agree that the Regals have nicer interiors. But then again they do cost more than similar GPs, don't they?
The GP SE prolly has the worst suspension of the GP brood - while the Regal LS (which I believe is the middle of the line model) is probably pretty well tuned.
I've only driven a regal once - and that was the Thrasher regal waaaay back when we had the first WDCCGP pulley day - but I don't recall a noticeable difference.
If it weren't for the heads up display I would certainly consider a Regal instead of a 4 door GP for my next car (well, GForce hopefully).