The Camaro. It has a little more power and shorter gearing. The 1998 Mustang
still had the rather regrettable non-split port 3.8L and it also has the 2.73 rear end gears. The 18 inch wheels just add more unsprung weight possibly make the effective gearing even taller with depending on tire choice. The CAI isn't going to help much as the stock intake wasn't a bottleneck to begin with.
The Camaros does weigh a bit more, but it has about 10 more HP and shorter gearing (which more than compensates for the Mustang's extra torque).