Both are old at this point. The Grand Prix is 18 years old & the Bonneville is 26 years old (hard to believe huh?). My parents used to own a '96 Bonneville. Speaking from personal experience it makes for a nice plush ride, but you'll also be sucking down slightly more gas but not as much as one would think. The Bonneville is only 200lbs more than the Grand Prix believe it or not & the gas mileage was actually rated the same as the Grand Prix in the city & 1 mpg less highway. My parents Bonneville did about 16-17mpg city & 27-28mpg highway when I've driven it. They never had any issues with it other than needing the intake manifold & gaskets & a blend door actuator. My '99 Grand Prix on the other hand I've had alot longer so some more issues have come through a handful being my own doing lol, but its been very good overall. Back when it was stock it was spot on to the 19 city/30 highway ratings. With mods about 17 city & I have seen as high as 32 highway.
Just some things to think about, at 60,000 miles your going to have to do the intake manifold gaskets on both if they haven't been done already. The Bonneville if its an SE is going to have rear drum brakes which don't perform as well in the wet weather. You really can't go wrong with either one as they are both great cars. I would have someone I trust look them over and pick the best of the two. Better yet if you know someone that can give you a carfax on both as well