Big difference between the two. Mr Norm offers a true Cuda conversion where they reshape the rear quarters and use actual 1971 Cuda tail lights. They will also modify the fenders to accept original fender gills. HXC only offers bolt-on parts, so the car will still look like a Challenger from the side. However, I think the HXC grille is way better than the Mr Norm's grille. If money were no object, I'd do the Mr Norm's conversion, then use the HXC grille. Even better is the Mr Norm's convertible, because the other features of the car that define it as a Challenger and not a Cuda, the back window and roof line, are no longer an issue when you cut the top off.
I got to see the Mr Norm's Cuda and Challenger droptops in person last weekend. I would totally buy the Cuda droptop. With the quarters and fenders modified I definitely think it looks like a Cuda and not just a modified Challenger. I would also buy the Challenger droptop. Looked sweet. After seeing these cars I would not just tack on the Cuda parts. I'd rather leave the Challenger alone or go full pull on the Cuda mod.