I was looking at getting one a while back and I noticed a few things.
1) Different front bumper. One has the lights stacked while the other has them horizontal.
2) I belive (may be wrong here) that the Formula did not come in a convertable or with T-tops (or one of the two...)
3) I think part was like the difference between a SSE and a SLE Bonneville. Different option group so they gave it a different model designation.
4) Aftermarket kits (such as the Firehawk) applied differently to the two models.
Not syaing this is all definate but it might help get some ideas going or the ball rolling...