I have used both the Sensatracs and KYB'* in my 1997 LeSabre (identical to a 1997 Bonneville except for body skins). The Sensatracs are not much firmer than stock -- they still allow the car to float like an old man highway cruiser. The KYB'* are MUCH MUCH firmer than stock. They transform the Bonneville/Lesabres from soft, sloppy handling float mobiles into very respectable machines. The 95-99 Bonneville uses the same strut cartridges and spring rates as the LeSabre, as well as the same part # sway bars for the standard and HD suspensions, so the rides are identical (which is pretty bad). I also upgraded from my 28mm front / 19mm rear stock sway bars to 32mm front/21 mm rear solid bars from a touring package Lesabre (will also fit the Bonnes). This also made a huge difference in how flat the car corners.
Bottom line is, the Sensatracs are designed for an OE ride quality, with valving that allows slightly increased damping at extreme compression/rebound. The car will still be a floater and have body roll. The KYB'* completely eliminate the OE float and roll.