Regals/ centurys are definately nice cars.
Engine= 3.8 vs 3.1
Inside: I believe all the regals had bucket seats and a center console. While every century I have seen has the front bench. Clusters are also a little different, century has 3 gauges, speed, relative temp and relative fuel. the rest is idiot lights. Radio selection is the same I believe. Both can be had with leather or cloth.
Suspension: I believe the century has slightly softer suspension, smaller sways, etc. Although I dont know what years changed with what. I know some regals have much stiffer suspension than the centurys.
Exterior: Regals have different front grills and the bumpers have a different molding design that goes around the car. Which makes the back bumper silightly different also. The headlights are the same for both. The regal taillights do not have the chrome strip that goes around the outside, instead it is solid plastic and painted black. Obviously wheel selection is slightly different as well. Most centurys came with 15" wheels while the regals got the GM standard 16". (225/60/16)
Over all very nice cars, not bad to drive and comfortable. Definately get a regal over the century, buckets are WAY more comfortable than that bench, the smaller sidewall on the 16 inch wheel makes such a difference, and it doesn't have the granny chrome accents on the outside.